My install went pretty smooth (backed up everything of course), then I copied the install files for 3.1.1 over top of my current install. Make sure to follow the install docs for each release - this will help with any changes you need to do in the web.config. Once I finshed the 3.1.1 install, I backed everything up again and then installed the latest version.

The only odd thing I found was that some old text/html modules ended up having no containers (just the content appeared, even in edit mode). It was pretty easy to fix - whatever page this happened on, I just added a new text/html module, opened its module settings, set the "show container" option to on and then applied the settings to all of the modules on the page. This fixed the old modules so that they now had containers. I'm sure there is probably a database fix for this as well, but we only had this problem on a few of our pages so the fix I did worked fine.

Good luck! Make sure to post back and let us know how it goes.