I have this problem on my site, but i've just noticed that webmasterworld is having the same problem too.
When someone presses the Tweet button the count goes up. That bit works okay. But when you then press the number in the speech bubble it's supposed to list all of the tweets that have been made. That is the bit that never works.
Here is a post at webmasterworld that currently shows 12 tweets -- [webmasterworld.com ] when you press '12' it doesnt find any tweets at all.