I have a 36 x 24" canvas print on my wall, taken with a D70 (6Mp). It looks fine - because you don't go and stick your nose up to it. It's all to do with distance, visual acuity and the angle subtended to the eye.
I restrict my printing requirements primarily to the meager 4X6 for family photo albums. I did, at the request of the family, do a large family portrait a while back,and it turned out very nice indeed. It is a 11" X 17" print which has an almost landscape like appearance to it. Standing right in front of it you cannot see any defects at all. It is very sharp, not just for it's size, I mean it IS sharp. Perfect. I would surmise from this that my D200 is capable of acceptable prints even larger, perhaps as mentioned even 20" X 30". At that size I have to imagine the quality is falling off somewhat, but after seeing the results of the 11" X 17" I have to think it would be a good looking 20" X 30".
After moderate P.P. in Paint Shop Pro XI, and making the 11 X 17 crop, I uploaded the file to Myphotopipe.com who made the 11 X 17 print. They actually did a large batch of them for me (one for each of my family members). They do non-standard sizes and pretty good size blow-ups as I recall. I bought the frames at Michael's (arts & crafts store). Go for it.
I'm a D80 user, likely to switch to a D200, but my D80 easily produces wondefully sharp, noise free, glossy print images up to 20x30. These are done by a commercial outfit, not at home. But the results are stunning. So your D200 will have no problem with this.
Thanks to all for the responses. I'm coming from a CP 990, and 8x10's were doable, so with three times the MP's, and a larger sensor, I was just wondering what will work. From the sound of the answers, with good technique, I can get prints as large as I would like.
Now, all I have to do is learn a decent digital work flow, to make my stuff worth printing.