I think we should prevent people from adding years which don't have 4 digits. There are a few problems that seem to pop up:
1. Some people pay no attention to the bit that says "YYYY" and enter the day in there.
2. Some people enter years but without the century included.
3. Some people make typos such as "201" instead of "2001"

There are a few artists from before the year 1000 but I think these are so uncommon (~13 at the moment) that requiring a zero-padded number would be acceptable. Right now, the edit form doesn't display leading zeros, so we would probably also need to change that, so that editing an artist with a date set doesn't cause a problem.

Some searches:
http://musicbrainz.org/search?query=begin%3A%5B0+TO+1000%5D&type=artist&limit=100&advanced=1 - 25 results, about half wrong
http://musicbrainz.org/search?query=begin%3A%5B0+TO+1000%5D&type=label&limit=100&advanced=1 - 2 results, all wrong
http://musicbrainz.org/search?query=date%3A%5B0+TO+1000%5D&type=release&limit=100&advanced=1 - 3 results, all wrong
There are also at least 4 relationships with years below 1000.