I use the official S2 case and have this week taken delivery of the official Samsung Car Dock. Had read on another forum that you could use it with the case but you can't really. It barely fits in there and is very likely to fall out when driving. To be honest its very easy to remove the case so I think I'm gonna keep it. Very nice bit of kit, if rather pricey.
There's a fair few options cheaper options on ebay. The only warning I would give you with those is that they won't charge the phone quickly enough when your using gps, ie the battery will constantly be draining. You don't get that issue with the official dock.