After upgrading from 2.1.4 to 2.2.2 I've had this problem a number of times, though not always (maybe 1 in 4).
After using a VM for a while (doesn't seem to matter what guest OS) I use Host-Q, pick "Save the machine state", watch the progress bar reach 100% and disappear. At this point the VM screen is still greyed and just sits there forever. I can minimize the VM with a Host tap, Alt-F9, and kill the VirtualBox processes. If I kill the VM process last I can later restore the VM successfully, indicating that the save worked.
Host is Ubuntu Jaunty 32 bit.
The last few lines of the log are:
00:06:39.323 Guest Log: VBOXNP: DLL unloaded.
00:06:40.872 Changing the VM state from 'RUNNING' to 'SUSPENDED'.
00:06:42.684 Changing the VM state from 'SUSPENDED' to 'SAVING'.
00:06:46.093 vboxClipboardConvertToUtf8ForX11: VBoxX11ClipboardReadVBoxData returned VERR_TIMEOUT
00:06:47.277 SharedFolders host service: saving state, u32ClientID = 1
00:06:47.704 Changing the VM state from 'SAVING' to 'SUSPENDED'.
00:06:47.715 Console::powerDown(): A request to power off the VM has been issued (mMachineState=9, InUninit=0)
00:06:47.720 SharedFolders host service: disconnected, u32ClientID = 1
00:06:47.720 VBoxX11ClipboardStopX11: stopping the shared clipboard X11 backend
00:11:47.757 VBoxX11ClipboardStopX11: rc=VERR_TIMEOUT