I've never quite understood the appeal of having your newsreader and mail program combined, but I suppose it makes sense for some workflows.
In any case, Mail's not going to do it (probably ever--I don't see Apple adding this), so you're either going to need to switch to Thunderbird or Entourage, or use Mail for mail and a different app for your news; Thunderbird is of course a decent option, but I'd probably stick to a dedicated newsreader personally. Thoth was amazing, but MT Newswatcher is free and handles text groups quite well, and there are others (have a look at VersionTracker). Unison is nice, but it's more binary centric, so it's probably not worth the money over the free options unless you really like the interface.