Ok, sharePoint doesn't use distribution lists. It only uses Security Groups, which many people mistake for distribution lists, because they behave like DLs (they can have email addresses and users). The "sync" with SharePoint groups is nothing more than adding an AD group to a SharePoint group. That way, the AD group gets whatever permissions the SharePoint group gets, so when you add or remove someone from the AD group, it carries through to SharePoint.
So, you can still use the AD group. Both the web services we're talking about actually come from SharePoint anyway and are populated by the SSP profile import, which pulls its data from AD. You can probably use that other web service, but I don't know how to leverage. I _do_ know how to leverage the one I mentioned. if you go try it, you'll see how easy it is. It gives a lot of info about the current user's memberships. It gives sharepoint site memberships, but those aren't group memberships, so don't get confused by those. Only worry about the ones that say DistributionList even though they are actually Security Groups. Confusing, huh?