Apparently since the kidnapping of the tourists in SW Egypt, the unofficial land crossing has not been possible (this info from 18 Oct 2008). There had been some travellers able to get across the border near Abu Simbel before then.
If you have a vehicle over 5metres long, you may have to hire the barge as we did. It is 24 metres by 6 metres wide, so can take a double decker bus plus several cars. The cost of the barge is 19,000 Egyptian pounds. If you manage to get a few cars on as well, this obviously reduces the cost, however we still ended up paying 14000 Egyptian. Probably wise to contact Mr Salah a little while before to make sure it's available. The barge does not allow ANY passengers, not even the driver, to stay on board.
I had been warned about steep ascent/descent ramps on the barge. Not a problem, the ramps were flat and there is the possibility of moving the barge at either dock to allow a flat drive on or off.
Lastly, the Wadi Halfa to Dongola road is fairly abysmal in places, however work is under way to tarmac the whole route. I would estimate that as of mid Oct 2008 it was approximately half completed, although only about 1/4 of the route was actually open. I'm guessing that within a year or so, it will be completed, but don't hold me to that. In the mean time, take sand boards and/or rely on the heavy roadmaking equipment to drag you out when you do get bogged down. The majority of the track is corrugated, hard packed sand. There are however sections of thick, very soft, fine sand that can catch you out. 4x4's are probably more sensible than double decker buses for this section of road.