I crossed Sudan from Ethiopia via Khartoum to Wadi halfa in August 2004 .It was really hot, but considering it is the Sahara its ok. Be aware of the fact that you can easily drink 5l of water a day and still be thirsty. All along the road are ceramic pots filled with water for travellers, this looks filthy, but i drank it every day without purifying and never got sick. I took some foolish chances crossing desert without mapping GPS, I dont advise this. One crossing from Dongola (I think) heading North off the road has markers which are easy to follow. Well worth the effort, on Mich Map its the thin black line joining a loop in the Nile. Roads are fun, but terrrrrrrible Corrrrrrugations will drive you made and break things you never thought would break.
As for security: Americans are unwelcome, no surprises there! I travelled without any hassles from police or soldiers and people were super friendly. Of course Darfur is to be avoided. US websites warn against travelling there, but they tend to be over cautious about countries they've recently bombed without good reasons.
Blue nile Sail Club in Khartoum is nice and a good place to meet others. If you run into a local tour guide/cyclist (youll know who i mean if you meet, he is there a lot) he is a fountain of knowledge and a really good guy. Wish i could remember his name.
"If we don't find something good at least we will find something new." Voltaire