Heres some info which I picked up on www.sahara-info.ch
the last couple of days.
I'm just going to summarise it as the internet in Bissau hasn't been working properly the last couple of days and I'm not taking any chances trying to do something fancy like transferring files.
The border crossing below is referred to as "Guelta Zemmour".
The Mauritanian Consul in Paderborn Germany (a german and very helpful and reliable) says that it was until the bombing in Casablanca possible for a short time although illegal. It is now only possible for UN workers. There was recently an attack on the train line just North of Choum and he doesn't recommend the area for tourists at the moment.
Another post is from a guy called Gebhard who I don't know. He doth sayeth; "we took the route via Atar,Choum,Bir Mogrein, Galat Zemmour. The route to Bir mog was no problem, at atar Choum and Bir Mog we were told that the border was open. At the moroccan border we were told that the border was closed since 01.05.2003 for civilians. After several hours of waiting we were taken in convoy to layounne where we did our entry formalities. We were only the third group to cross the border since 01.05."
So maybe the crossing west to east is impossible and the other way round it is, who knows, as Chris says if a lot of people ask maybe they will think it over although I must admit that security has been stepped up in the south since the bombings in Casa.