First, I must say that I really am excited about this product! I just can't seem to get enough of what it can do for such a reasonable price! I use Ubuntu 5.10 as my primary OS and WinXP as the guest. I can create a share on the guest and the primary can see it. Can I go the other-way-around? Are there any plans to add native file sharing (i.e. no network shares) in the near future?
You should use SAMBA to share from Ubuntu. Try looking in the Configuration Editor under system, smb to set the workgroup and under the System menu, Administration, Shared Folders to set the share.