So I was thoroughly annoyed to find a guest account with a sandboxed Safari after I had setup iCloud on OS X 10.7.2. Turns out, this only appears when you have "Find My Mac" enabled, and it does make sense (how can your mac be found if no one is using it?). If you don't like the idea of a guest account on your mac (even if you've disabled the guest account in System Preferences) make sure that Find My Mac is turned off.
Strangely enough, if you have FileVault enabled for full-disk encryption, the guest account login still shows up when the system starts. This leads me to believe that some of the system preferences and a copy of Safari are stored in the Lion recovery partition. Annoyingly enough, this includes keys for wireless access points. I don't know if there is a way to get that information (I suspect not, but you never know), but this means that some things are being stored outside of the FileVault, which concerns me somewhat.