Most feature films are converted to IMAX format using a process called DMR. (which somehow is upposed to stand for digital remastering). Blowing up an unaltered 35mm image to the IMAX sceeen results in a blurry picture that is too dark and with colors washed out. Kinda looks like you’re watchign a TV station that doesn’t tune in completely. The DMR process interpolates the image to remove all of those artifacts.
The answer to the OP’s question is that it varies from film to film. In some instances, they have changed the aspect ratio. Sometimes they show it in a letterbox format with the original aspect ratio. Never pan and scan.
A lot of films have been made specifically for the IMAX format, but to my knowledge, just 13 feature films have been converted for IMAX, including Batman Begins two weeks ago. Two others (Charlie and the CHocolate Factory and the new Harry Potter film) are slated for IMAX release later this year.
There are some websites and message boards devoted to discussing IMAX films, and the quality of the DMR releases is a pretty hot topic.