You are not imagining it - the number of trailers previewing "coming attractions" before feature presentations has been increasing.
Ten years ago there were two to four trailers playing before film screenings. Today, about six or more trailers precede film screenings in North America- sometimes after several commercials for consumer-products - as well as 30-second "teasers."
The US's National Association of Theatre Owners doesn't limit the number of trailers, but prefers that that nothing interfere with the filmgoers' experience. Some smaller cinema chains in the US restrict the trailers to three per film and will not play ads.
One of the reasons that theaters will play more trailers is that operators are paid to show them. This practice has been done internationally for several years but is fairly new in the US.
Since trailers are effective in building awareness, interest and anticipation for upcoming films, studios usually attach one trailer to each film, promoting another film from the same company.
With Iron Man 2, for instance, there are a couple extra trailers of other Paramount Pictures included. Exhibitors are now asking for compensation for additional trailers which can occur in various ways.
The MPAA (Motion Picture Association of America) guidelines for trailers sets a maximum length of 2.5 minutes with occasional exceptions.
The question is whether film trailers are considered entertainment or marketing by the audience.