I'm using the WP 3.0.1 and My Calendar version 1.11.2.
When I click on the "Next month" link on the month view of the calendar (full view), calendar for new month apears, but pop-ups for event description do not work.
I have noticed same malfunction with mini calendar (used in php code, as short code or as a widget in sidebar).
If I use dropdown menu to select month it works as it should.
Btw, I have tested this with my own theme and WP Twenty Ten 1.1 theme. So I believe this cannot be my fault in any way.