When Microsoft introduced the ribbon interface, it offered up a series of guides showing where familiar commands had moved to. The ribbon is even more pervasive in Office 2010, so Microsoft has updated those guides ahead of the newest suite’s release.
The Ribbon interface isn't to everyone's taste (mine included), but the mapping guides are one of the easiest ways to learn where stuff is located in the newest version. Click on the menu command from Office 2003, and you're shown where to find it in the new 2010 release.
Right now, there are mappings for Word, Excel and PowerPoint, but guides for the other Office 2010 apps are promised in the future. One note: unsurprisingly, the guides seem to work best when accessed via Internet Explorer (I had no luck with either Firefox or Chrome).