I don't know if it's something I didn't configure correctly, but I discovered a somewhat serious problem with the shared folder facility on an Ubuntu Hardy 64 host. I installed the 64 bit version of the non open source VirtualBox software, running an XP client.
The shared folders appear to work, albeit there are problems in the XP client in mapping to them, but that's not the issue I'm addressing. Basically, when shared folders are mapped, it takes a very long time for XP to, say, open an Explorer window to look at them. Opening and saving documents in Word, for instance, is time consuming and frustrating. And worst of all, it seems to affect the integrity of the Ubuntu host system itself, causing slowness and a variety of unexplainable problems with things getting stuck.
I did away with all of the shared folders and circumvented the problem by configuring Samba on the host and mapping to it from the client. This works fine and all of the problems I described seem to have gone away.