I've done several prints in the 24" range, and they look great off my 1D3. I did have to learn to start framing tighter when I got the 1D3, after being used to the T2i's 18MP. The 1D3s files produce images so much sharper though.
If I was going to upgrade from the 1D3, I don't think I could go anywhere but more 1D bodies. I've not yet started bouncing my head off the ceiling on the 1D3's capabilities, so upgrading the body is a long way off for me.
The 1D3 limitations that impact my shooting would be sensor resolution, ISO capability, and LCD quality. I was able to get over the LCD thing on the first day -- I thought it was taking horrible photos, but looked on my PC and was floored with the high quality. LCD is now for checking histogram, composition, and changing all the wonderful settings.
Sensor resolution is in the back of my mind, but it so rarely bins a photo. I have SO many more keepers on my 1D3 than I did on my T2i.
ISO performance, isn't really a problem, but I know the newer high-end bodies can do a lot more. As it is, the ISO performance of the 1D3 is still better than my T2i at the same settings, so it's still an improvement for me.
I also agree on the 1D3 battery life. I shot 3K photos for a swim team, and went home with a 40% charge. On a 5-year-old battery. Insane.
So, for me, I'm happy right now with how much money I have tied up in bodies, and what that money has bought me. I'm ready to start working on glass again.