I must admit I'm a bit surprised that this slipped past me, so I hope I'm not the only one that didn't realize Entourage can sync with the iPhone via its oft-forgotten ability to play well with Mac OS X's Sync Services. According to this Apple support doc, switching on the preference you see above in Entourage 11.3.5 or later will break down the barriers between Entourage, Address Book and iCal, allowing iTunes to unite your iPhone and Entourage in beautiful synching harmony (however, as far as I can tell, iTunes won't be able to sync your actual email accounts from Entourage).
How well this bridge will work in practice I can't tell you; I'm not an Entourage man myself. But this might be great news for those still on the fence between an iPhone and something like a BlackBerry for whom Entourage synching is a deal maker or breaker. If anyone has been using their iPhone with Entourage, please sound off on how this setup's been treatin' you.