The USMLE (your med school licensing exam and good residency pre-req) has been computerized for the last five years. The computer format isn't bad once your used to it. The fact that you can re-schedule it at will is a major boon (no more, "I was sick the day I took it" excuses for bad performance though). Also, not having to have controlled twenty or thirty minute sit and listen to instructions periods or controlled breaks are positives. If the MCAT is made computer adapative, like the GRE, the MCAT may also be made a bit shorter in duration as a more accurate assessment of your abilities can be made more quickly. I think the only problem with the computer testing is if you have someone doing a writing section next to you while you are doing some other section-- loud typing can be rather distracting, but you can get annoying people next to you with a pen and paper version too.
You will be a guinea pig if you take it the first couple of times it's offered.