It says in the FAQ that if you edit the question you may get the question reopened.
How often happens that a closed question after being edited gets reopened?
Using data.stackexchange.com I see over 100 times per month using this query:
Here's the monthly data for reopens in 2012:
And for closes:
So, it appears to be roughly 1-2% that get reopened these days, versus a 3% overall reopen to close ratio (3079 total reopens, 94993 total closes) across the entire database.