Is it possible to exercise too much?

bump...would love to know the answer to this question also!

Well there is certainly such a thing as overtraining, but I am not sure where the line is to be drawn.

http://en.wikipedia.org/wiki/Overtraining

Definitely. Too much exercise normally leads to injury.
One thing to monitor is your Resting Heart Rate, if it is getting too high above the normal you need to rest.

Yes, you can overtrain your body. This is where mixing up routines is SO important. Doing the same thing over & over will work the same muscles/groups and sooner or later rather than building they will start to break down. When they break down it takes twice as much work to build them back up (I'm speaking from experience, I'm building mine back up to where they were 4 months ago due to pushing too hard - have family that are PT's & PFT's).
While I do double workouts most days, I've upped the cardio & alternate weight-training between body portions (torso or legs) and Abs. I do not work the same muscle groups with weight/Ab training 2 days in a row, and when I need a break without taking a break I add in T'ai Chi or Yoga for my evening workout instead of weights/strength training . You can still get an incredible burn and both help train sub-muscles creating the long, lean look most would like (unless you're looking for bulk).

From what you're doing though, unless you're noticing swelling/fluid retention in your joints, extreme hunger or lack there-of, you're probably OK. Just don't forget to take your rest days (at least once a week even I take off from my workouts - I may still clean the house/do laundry or car work, but I'm not intentionally "working" muscles )

Yes. But it depends on a lot of individual factors like what you are used to and how quickly you add all that exercise into your life and how much sleep you get on a regular basis. Some people can only be motivated when they are really out of shape by going "all out" with their routines. The thing to keep in mind though is that if you go all out for 60 or 90 or even 365 days and then lose all enthusiasm that you run the risk of ending up in a viscous cycle. It really depends on your goals. I once trained about 8 months for a marathon and was working out 6 days a week with lots of time and distance put in. Though I did succeed in my goal, it wasn't something I wanted to do year after year. Some people enjoy "endurance sports" like triatholon and marathons though and will be happy to put in hours of excercise day-after-day week-after-week for decades. That's certainly commendable, but the average person doesn't need that kind of commitment to stay healthy.

I have come to the personal conclusion that CONSISTENCY trumps VOLUME no matter what kind of excercise you do especially if (like me) you work a sedentary job and don't get the proper amount of daily activity without excercising. Pay attention to your body and to how you feel and don't feel too much pressure to put in X amount of hours or burn X amount of calories just because you need to meet a goal. If you are being consistent with both your diet and your exercise routine most of the time then over time (and by most I really mean like 85% not 51%) you will reach your goals and be able to maintain a healthy weght and lifestyle. Also don't fall into the trap of thinking that excercise is more important than diet. I can achieve a net loss of 300 calories by not eating a slice of pizza for lunch and not spend a single minute excercising!

If you are doing anabolic workouts like lifting weights than rest is even more important because its an integral part of the process.

Also highly second the comments on mixing up your routines. Not only will it help you develop in your goals faster (by avoiding adaptation) but it will keep you from getting mentally bored with your workouts.