You can check out all of your applications to see if you're running any PowerPC software. System Profiler -> Applications -> sort by Kind. Only culprit on my system is some random CS4 stuff.
Combine that with requiring CS5, and Lion will be a very expensive upgrade for many people.
http://en.wikipedia.org/wiki/Microsoft_Office_2008_for_Mac#R...
If you didn't use Terminal to install Office, then you must have either upgraded Snow Leopard instead of doing a clean install, which would allow you to use Office without running the installer again, or you have Rosetta installed without realizing it.