Running at his local track in Southampton, the 23-year-old Welshman was approached by a club-mate who said he had bet pounds 40 on him to win this summer's World Championships.
"This was a guy who worked in a factory, and he probably couldn't afford to bet so much money," Thomas said.
A bit worrying, perhaps? All adding to the pressure of expectation? That is not how this outgoing character views life.
"I would be so proud to win it for people like him," he said. "They have so much faith in me."
That faith has been generated by Thomas' impressive record since becoming a full-time athlete 18 months ago.
At the Olympics he won silver as one of the 400m relay team and finished fifth in the individual final. This season, in an event which has wrought havoc on the health of a succession of British athletes, he has gone from strength to strength.
After a second successive winter of training in South Africa, he defeated all his main domestic rivals last Sunday week in Sheffield, where he ran a personal best of 44.49sec. Three days later, in a rainswept Lausanne grand prix, he took 0.03 off that time in finishing second to the world record holder, Butch Reynolds.
The American will not be at the World Championships, because he failed to qualify in the US trials. The world and Olympic 400m champion, Michael Johnson, also failed to qualify because he missed the trials with injury, and his only chance of being in Athens next month rests with the rumour that he, along with other defending champions, will be offered a wild card.
With Roger Black, Britain's Olympic silver medallist, hampered by a virus, Thomas's prospects are suddenly dizzying. He heads this season's 400m rankings.
Interviewed recently for Channel 4's Saturday magazine programme, Thomas was asked how he felt about becoming Britain's biggest star. "I just laughed," he said. "I don't think of myself as a star. It's scary when I get told that I am the fastest 400m runner this year. But I'm not going to let it bother me. I haven't said to anyone I'm going to win."
This evening in Birmingham, Thomas runs in the opening round of the British world championships trials, where he faces two of his fellow relay silver medallists from Atlanta - Jamie Baulch, his perennial Welsh rival, and Mark Richardson, who finished a place behind him in Sheffield and Lausanne.
"I want to win the trial, but I wouldn't be shocked if I was beaten by Mark," Thomas said. "He is an awesome talent. Going to Athens is what's important. When I get there, I'll give it some."
Thomas's coach, Mike Smith, has guided the careers of a series of outstanding 400m runners, including Donna Murray, Todd Bennett, Kriss Akabusi and Black.
He believes Thomas can break Black's British record of 44.37sec this season. "He might have had it in Sheffield if he hadn't put his arms up two metres from the line," Smith said.
"And I think he can get an individual medal at the World Championships. At this moment, Iwan is the fastest man over 400m, and, if all goes well, he could almost be looking at gold. I say that with hesitancy because that is almost the kiss of death."
Smith, now 68, took early retirement from his teaching job in Southampton, but his committment to coaching athletes locally has been unwavering.
Thomas started training with his group in 1994 when he was completing his degree in sports studies and leisure management at the West London Institute. A year later he moved to Southampton to devote himself to training. He was financially supported by his mother Ann, a teacher, and father Cliff, a former Group Captain in the RAF, who now works as an aviation consultant.
"They bought me a flat and provided me with my food," he said. "They gave me a year. They would have given me more. But I am financially independent now.
"If it wasn't for my parents and Mike I wouldn't be where I am today. Mike's quite a strange character - very grumpy. He hardly ever compliments me on my running. He'll say, `not bad, not bad.' But I think he doesn't want me to become complacent.
"I don't know exactly how he does it, but he's got me in perfect shape at the right time."
Before the race in Sheffield, Britain's coaching director, Malcolm Arnold, asked Thomas if he was going to win. "I looked him in the eyes and said: `Yes.' I was so up for it."
His eagerness had been heightened by the events of the spring, when Baulch earned all the headlines for indoor performances which culminated in a world indoor silver medal.
For Thomas, whose individual performance in Atlanta had been overshadowed by Black's, it was not an easy time. His height and powerful build are not suited to indoor running, but he had to be persuaded to concentrate on his outdoor preparation.
"I found it hard when Jamie got all the publicity," Thomas said. "I felt a bit left out in the cold, training on my own."
What has worked in his favour, Smith feels, is an uncomplicated approach to running. "Iwan is an easy-going character. He has this directness about him, which is a gift. You have to jolly him along and act like his Dad at times. But he does everything anyone wants of him. He puts his back into the work.
"He can be a bit of an idiot sometimes. A bit of a scatterbrain. Before he went full-time he was very much the student. Sometimes he would get down to train, sometimes he wouldn't. You could have written a book of the excuses he used. His car had broken down, or someone had thumped him in a pub... But once he moved down, all that casual attitude died off."
Eleven years ago, Black won Commonwealth and European titles. Now Smith has another talent poised for great things.
"Iwan hasn't got the 100 and 200m speed that Roger has, but he is much stronger," said Smith, who has already made plans to travel out to Athens - his first foreign trip in five years. "He's not - touching wood, and my head, - so prone to injury as Roger has been."
For Thomas, the going is indeed good right now. "I'm just really happy with life," he said. "Everything is going well." Long may it continue.Reuse content