[ambition]

It's a good idea, and I think it would make an excellent supplement to current interview practices. There would be some difficult logistical issues: Unfamiliar dev environment, etc. It wouldn't replace whiteboard coding, since you lose the ability to test for culture fit, give hints, examine thought process, ask about small decisions, get a gut check and other aspects of whiteboard coding that have nothing to do with the specific question asked.

Some companies ask for a small code sample "that you're proud of" in advance of onsite interviews, I think that's another good practice.