Nikon's newly announced D40 significantly lowers the cost of a DSLR. The Nikon D40 is small, light and undeniably cheap at just $599/£449.99/€679.99 with the new 18-55mm kit lens (it won’t be sold as body-only). This pricing puts the D40 $300 below the D50. This new 18-55mm kit lens is a updated version of Nikon's 18-55mm lens improved for the D40. As usual Phil Askey has and in depth review of the D40 over at www.dpreview.com.
I think Real estate photographers using the D40 would get the best result if they added a wide-angle lens. 18mm with a 1.5x field of view crop (effectively 27mm) is on the ragged edge of what you can get by with shooting interiors. Be careful though when choosing a lens for the D40 it can only auto focus with lenses that have built-in focus motors.
As with any DSLR being used for interiors, I'd also recommend a external flash unit to give you the power to light a room nicely. Nikon has designed a new external flash (the SB-400) for the D40. The SB-400 unit has a guide number of 21 (m at ISO 100 at 18mm) and has a shooting distance of 60 cm to 20 m (2 to 66 ft).