Some results of testing GNOME's at-spi framework using dbus compared to ORBit are now available. You can check them
out http://live.gnome.org/GAP/AtSpiDbusInvestigation. Quoting Mark Doffman:

GOK and Orca were profiled to get a good idea of the type of traffic on the AT-SPI interface. Using this information the performance of some of the most common method calls were tested in D-Bus and ORBit.

D-Bus is undoubtedly slower at most of the common method calls, 5-6x slower when making a call that passes one int as an argument. When passing more data per call this speed difference decreases. ORBit takes a long time to pass an Object reference, making D-Bus up to 1.5x faster at these method calls.

Although D-Bus is the slower transport, looking at the calls made by Orca and GOK, we feel it will be possible to provide sensible caching that should mitigate this effect.

If you are willing to help out, and have any thoughts on the way forward, I would strongly encourage you to check out the wiki page and get involved. If the change over to dbus ends up going ahead, we may finally see KDE get some accessibility, something that I have been waiting a long time for.