I think it is a pretty safe bet that following the same workout regimen for too long will limit your body�s upside over time. The body becomes accustomed to the same workouts and needs change to shock the system.
For about 3 months, I had a regimen of 4 day cycles for strength training. One day back and calves, one day squats and arms, one day hams and shoulders, and one day chest. I would throw in cardio, abs, and an occasional day off. At the beginning of the three months, there were noticeable gains in appearance and strength. However, by the end, I was putting in my time at the gym and just not seeing the same types of gains.
Obviously, I figured this meant it was time to change (in actuality, I was probably a month behind). Now, I have switched things up a bit. I take one week and alternate between 2 major muscle groups. For example, last week was back one day, alternated with ham days. This week, today will be chest, tomorrow squats, and Wed. back to chest.
So far, I have been doing this for close to a month, and I definitely feel stronger and see some visible gains.

Another thing I have incorporated into my "workout week" is the "300 workout." Yes, I am jumping on the bandwagon, and nearly every single fitness related site I have visited lately has something about this workout. Let me tell you, there is a reason for this. I haven't seen this movie yet (partly because of the concern I have with its historical inaccuracy, which is neither he nor there), but I do know that these actors have some SICK bods.
Wow. I have only done it a few days, but it just kicks butt. It is extremely intense, and part of the objective is to finish it fast. The main 300 workout that I have seen is like this:
25 pullups
50 deadlifts at 135lbs.
50 pushups
50 jumps on to a 2 ft. platform
50 floor wipers at 135lbs
50 kettlebell clean and jerk (25 with each arm)
25 pullups
I have been throwing in some extra exercises as part of this circuit, including 50 squats with 135 lbs., 50 curls with a 45 lb bar, and 50 dips.
The best part is, the workout takes about 20 minutes, with some additional time for the extra things I throw in. Adding in about 20 minutes of abs and a half hour of cardio, and that is a pretty great, fun workout that really changes things up.