I know this is too late, but try the web service UserGroup.asmx, then select GetUserInfo. Andy Bonner had a blog about this, which is archived here:
http://cs.vbcity.com/blogs/skullcrusher/archive/2008/09/26/get-the-full-name-in-a-web-hosted-infopath-form.aspx
While Andy's blog details how to get the full name, you can see it should also be ...