Hmmm... I have two indoor cats who just aren't good with the big, bad, outside world. One cat got out just before hurricane Hugo hit when he was living on the coast. He was gone for two weeks and we thought he was a gonner. One day he turned up sitting on a neighbors car - he's not been an outside cat since. He has always tried to sneak out whenever the door opens but he never tends to go far. Now that he's almost 18 years old, i think he only sneaks out to prove that he still can since he just sorta stands on the porch when it does happen.
My other cat was hit by a car as a kitten (that's how i got him - friend of the vet who spared him and fixed him up). He's very scared of the outside world and hasn't ever really wanted to go out. That's changing as we live in a place with LOADS of birds and squirrels that come onto the deck. I think he's getting curious.
However, we have a breeding pair of red-shouldered hawks out back so i really don't like to let them out at all! The hawks have attacked one or two cats in the neighborhood already, so when i do let my cats out on the deck it's ONLY under supervision... Once, my older, slower cat got out while the hawks were screeching and circling overhead. I about had a breakdown trying to get that rasty old man cat indoors. (I LOVE the hawks but don't feel the need to feed them like i do the other birds) Also i do like the fact that the cats don't have ticks or a chance for contracting Leukemia or Feline Immunodeficiency Virus (FIV) as readily. I was a researcher on a FIV (Feline AIDS) project when i was in college and the affects are horrible.
Ok, saying all of that, my aunt is taking care of a ferral cat colony behind her house and they are always outside. They seem to be doing quite well but get in quite a few skirmishes which require attention. Two are indoor/outdoor cats, one of which would prefer to be indoor only. I hate having to check them all the time for ticks but they're usually pretty clean.
But overall i don't think indoor cats are necessarily less healthy - my old geezer is very healthy. Perhaps they don't have to eat as much (less excercise) but they don't seem to be missing out.