Looking at XOrg's log, I see that there is some ABI mismatch, which causes VirtualBox's video driver fail loading:
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[ 24.672] (II) Module vboxvideo: vendor="Oracle Corporation"
[ 24.672] compiled for 126.96.36.1991, module version = 1.0.1
[ 24.672] Module class: X.Org Video Driver
[ 24.672] ABI class: X.Org Video Driver, version 9.0
[ 24.672] (EE) module ABI major version (9) doesn't match the server's version (10)
[ 24.672] (II) UnloadModule: "vboxvideo"
[ 24.672] (II) Unloading vboxvideo
[ 24.672] (EE) Failed to load module "vboxvideo" (module requirement mismatch, 0)
I get the same problem with Fedora 15 pre-release as well (which seems to have the same XOrg server). I do realise these are pre-release software, but just checking if anyone knows any workarounds or if support will be coming soon.