I recently sold a 12MP image taken with a D300 using the Nikon 10.5 fisheye and enlarged to to 30x60. After initial editing of a JPG with Capture NX2, It took careful noise reduction with Noiseware Pro standalone, enlarging with Perfect Resize 7 and sharpening with Sharpener Pro 2. I had it printed by BayPhoto on Metallic paper.
The client was very pleased with the quality, as they were very concerned about degraded image quality, especially pixelation. If you stand a foot away from the image you'll see some, but at a reasonable viewing distance it looks sharp and noisefree.