Microsoft has published a new knowledgebase article outlining the known issues with Office for Mac running under Mac OS X 10.7 Lion: http://support.microsoft.com/kb/2586538
Highlights: Office 2004 and older won’t run because they require Rosetta, and some Office 2008/2011 applications may crash when doing things like moving spreadsheets between workbooks in Excel, or pressing command-tab in PowerPoint to switch to presentation mode. Also noteworthy is “you may not be able to import message from Lion Mail” into either Outlook or Entourage.
If you plan on running Office 2008 or 2011 in Lion, reading through this Microsoft knowledgebase article should be considered required reading. It should also be noted that although Office 2011 doesn’t support Lion’s Auto Save, Versions, and full-screen mode features… Microsoft is working on a major update that will add these features in a “few months”.