Microsoft has released an article about Office compatibility with Lion
You can read it at: http://support.microsoft.com/kb/2586538
In addition to the total incompatibility with Office 2004, Microsoft have identified numerous issues with 2008 and 2011. You can expect that Microsoft will release an update within the next couple of weeks.
If you use Office extensively for work, you may wish to bear these things in mind before upgrading!
Cheers,
Rodney