My wife and I just got back into riding bicycles. We're both in our mid 30s and haven't ridden since we were kids. Our reasons for getting into biking are very different, though.
My wife is a marathon runner and wants to get into doing triathlons. We ended up getting a hybrid ('07 Trek 7.2FX) to get her started. She's only had the bike for a few days and she's already loving it. You can't beat the aerobic workout without all of the pounding on your joints.
I'm more into motorsports like riding motorcycles and driving cars on a road course. Power-to-weight is a big part of those 2 hobbies, so, at 5'4" and 195 lbs., I could benefit from losing a pound or thirty. I ended up getting a cyclocross bike ('07 Trek X01, should be here anyday now). My general goal is to trim down and work on my cardio. Specifically, I'd like to be fit enough to do the Bike-to-Work Day this May. That's a tall order for me since my commute is ~45 miles each way. You always gotta aim high, I figure. I hope to hit some light trails with some buddies as well.