MS updated their ie testing images on Thursday. ie8 + XP happens to be one of them. So I'd assume it'll work under Parallels. You'll have to run them thru Transporter first.
http://www.microsoft.com/downloads/details.aspx?FamilyId=21E...
http://www.microsoft.com/downloads/details.aspx?FamilyId=21E...
Ahem.
On a completely unrelated note: Does anybody know if IE 8 runs in XP under Parallels? I have some websites that I should probably check.