Here are the iOS version statistics for April 14th 2012. The poll was taken over March 14th to April 13th.
As you can see, the numbers are dramatically showing that 5.1 is now king. Since 5.1 came out on March 7th, it is reasonable to think that there were still a few users with 5.0 in the first few weeks so looking at just the last 2 weeks in April, 5.1 is at 80% and 5.0 at 16%.
UPDATE: You can view the latest stats here