I usually work my arms and chest, just because my legs are naturally strong. Right now I'm not allowed to workout (damn broken L5 and herniated L5-S1, L4-L5, and L3-L4 - yeah, I'm only 27)... but when I do it's usally something like this:
Day 1: Cardio (20 min bike, 40 min eliptical), Arms
Day 2: Cardio (20 min bike, 20 min eliptical), Chest/Abs, Cardio (20 min treadmill)
Day 3: Cardio (20 min bike, 40 min eliptical), Legs, Cardio (10 min treadmill)
Day 4: Cardio (20 min bike, 20 min eliptical), Arms, Cardio (20 min treadmill)
Day 5: Cardio (20 min bike, 40 min eliptical), Chest/Abs
Day 6 and Day 7: off
It worked for me, it helped me lose weight, trim everywhere, and put on a little bit (but not so much that its ugly) of defined muscle. The bike gets me warmed up, the eliptical gets my heart pumping, and the treadmill at the end two days a week gives me a little something different, and helps me cool down a bit.