I shared a directory on my hostnode/local system (running Ubuntu 10.04) with my Guest system running Windows XP.  I have no idea why it's not mentioned or documented in an obvious way, but in the Windows client you just access it with "\\Vboxsvr"

Once you access that share you'll have access to all of the VBOX shares on your local host.  I think it should indicate it somewhere when you enable the sharing.  Yes, I'm sure it's buried somewhere pages deep in a wiki and I believe I found the answer first on a Ubuntu forum.