I uninstalled the beta using the uninstaller in the installation package and went back to Safari 2, and now MSN Messenger works again.
[robg adds: A Google search on this topic seems to indicate this is not an isolated incident, but I haven't verified it myself. A comment on ">this blog entry suggests that switching from Safari to another app to MSN Messenger solves that crashing problem; if someone tries that, please post your results.]

