Using the Blogs MU theme, however this issue is present on all themes I have tested in an effort to isolate the problem, including Twenty Ten.
That being said, the BP Activity Stream is essentially completely without function. All you can do is post a status update. All other actions on the BP Activity Stream just show the Ajax spinner, indefinitely, with no action. Needless to say, this is a serious issue within an active community, leading to incessant support tickets about the issue, etc.