There really can't be any guarantees unless the developer states they'll support Windows 8. There's always been "little" things that creep up in the past, like some crashes that would happen on Windows 7 but not Windows Vista / XP for a game developed before Windows 7. As botat29 mentioned, drivers also play a large role, and you'll obviously have less driver versions to work with on 8 than on 7 for now. (e.g. You can't revert to older drivers if something in the newest driver breaks your older game.)
That said, I'm trying out Windows 8 RTM evaluation (see
Windows Metro Style Apps Developer Downloads) and do plan to use it on my main computer as I want to try developing apps on it.