Ok, after some searching, I think I've found the source of the problem: VBoxClient failed to start at login, probably because it couldn't access /dev/vboxadd, which is only accessible by root:root.
From the VirtualBox site and changelog.Debian.gz I gathered that VirtualBox 2.0.0 and up are built with 'hardening' enabled by default[1]. which requires all binaries to be setuid root. Changing the VBoxClient to be setuid root and restarting the guest solved my problem.
Ok, after some searching, I think I've found the source of the problem: VBoxClient failed to start at login, probably because it couldn't access /dev/vboxadd, which is only accessible by root:root.
From the VirtualBox site and changelog.Debian.gz I gathered that VirtualBox 2.0.0 and up are built with 'hardening' enabled by default[1]. which requires all binaries to be setuid root. Changing the VBoxClient to be setuid root and restarting the guest solved my problem.
[1]: See configure, lines 68, 2131-2149
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