i can't be sure about the retail version of Office 2010, but I know that with the Beta version users were offered a choice of Upgrade over Customize.
The Upgrade removed Office 2007 and replaced it with 2010, so be sure you have the 2007 setup disks if ever you want to revert back.
The Customize option allows you to have both Office versions installed at the same time, in the Beta version. More about that here;
I hope that helps.
Just got my copy of 32-bit Office Pro Plus from school, and before I install it, I just wanted to see if there have been any issues doing an in-place upgrade to 2010 as yet.