I'm not sure what Spring offers, but the Treo 650 may be your only solution.
The Treo 650 works "ok" with iSync. You can only sync one address (Home only). You can't sync category names. You cannot sync the Memo Pad (although you can fix it with Missing Sync). But if you can live with all of the idiosyncrasies of the way Palm OS works with Mac OSX, it is a pretty good solution.
I'm not a fan of the Treo, but since you're committed to Sprint, it might be your best, if not only, solution.