There's a new Microsoft Security Advisory (973472) to review while waiting for tomorrow's Microsoft Updates.
A vulnerability exists in the ActiveX control used by, wait for it… Internet Explorer to display Excel spreadsheets (Office Web Components). It doesn't affect Office 2007 but many other versions are vulnerable.
Considering the timing of this advisory, it isn't likely to be patched tomorrow, so check out the workaround instructions.
Microsoft states that they're aware of "attacks attempting to exploit the vulnerability".