Thin cases with port covers don't exist.
I don't think having them exposed is an issue, but you could always try these out: Amazon.com: Cosmos Black silicone anti-dust stopper/plug set - 4x USB, 2x 3.5mm earphone/mic jack: Electronics
That's probably the only thing I'll miss on my Fascinate, the little sliding door that covers the USB hookup.
I hate those port covers... they always get in the way, but that is just a personal preference.
Aegis Case for Motorola DROID RAZR MAXX HD
Is it slim enough for you??