Nikon D5100 + SU-800 = No onboard flash?

I'm considering getting a D5100 but I'm concerned about the flash limitations. I have an SB-600 and will at times want to use this off-camera for macro work. The problem is that I like to use SB-600 off-camera with my D90 AND use onboard flash. I know that there are various ways to achieve D5100 + SB-600 off-camera (TTL cords, SU-800, radio transmitters etc) but I assume that as all of these gadgets attach to the hotshoe of the D5100, the camera will automatically lock the onboard flash from use?

Yeah, no flash in that case:/

Rather than a SU-800 find a SB-700, SB-800 or SB-900 and use that on camera, these flashes have a 'commander' mode and will control other off camera flashes in the Nikon CLS system. Pricing should be similar to the SU-800.