Data source: March 14, 2012 SEDE data: SEDE querySEDE query
This analysis can be run against the public data dumps, but the query is very very heavy. (Read: best of luck running against Stack Overflow.) The SEDE query is herehere if you want to run against other sites (you may need to add a minimum threshold for the combinations for the results to be meaningful).
As with the previous query, this can run against the public dumps, but you have to limit the number of combinations for the bigger sites. The SEDE query is herehere.