Indeed, I am using Fences with Windows 7 RC 64 bits.
I am not sure if there is "official" support Fen ces under Windows 7, specially conidering that fences itself is still a beta product. Anyway, what I've seen is that, in thepast, Jeff has helped us Windows 7 testers. (Specially when it was an invitation only beta).
Doing some tests I´ve found that the problem is indeed related to personallizing Windows 7 background, and somehow related to Aero enabled themes. If I select any "Basic and High Contrast Theme" fences would work just fine, but if I pick any "Aero Theme" then fences would not show properly. I would add that this is not something that occurs on a completely repetitive manner... switching between Aero and non-aero themes would sometimes yield an operable Fences (when the Aero theme is selected again), but only a few times... I have to switch between them multiple times to get a functional fences setup.
Something to note is that I did not perceived this problem but until very recently (a couple of days ago), which happens to coincde with the installation of the latest WindowBlinds beta for Windows 7. But I am not sure if it might be somehow related to this problem.