I just got back from Siem Reap yesterday (22 Sep 09) and am in the process of compiling a Trip Report which will take a few days.
Briefly though the road from Chong Sa Ngam to Anlong Veng to Siem Reap is excellent with good sealed surface and gentle curves and not much traffic. Takes about 2 hrs
The big problem though is how to get to it. Unless you have prior Customs consent, Cambodian Customs wont let you in at Chong Sa Ngam/Choam so you have to go to Pak Choeng/Osmach and they may let you through. After some haggling I got through after being refused at Chong Sa Ngam, but it was by no means a foregone conclusion. No problem getting out of Thailand at either place. Gets a bit complicated when you get stamped out of Thailand personal and bike and then get refused entry into Cambodia. You have to get Unstamped with all the questions that raises in your Passport.
Then you have to get to Anlong Veng about 70 Km of shocking unsealed pot holed slippery track. After that its a breeze.
Normally at Poipet and Hat Lek it's the Thais who make the paperwork and the Khmers don't even look at the vehicle..... At O'Smach from Surin the Khmers kick up trouble on bringing in a bike? What do they ask for? I ride a Thai registered bike.
I ride a Cambodian registered bike so that may make a difference. But I never had any problem entering Chong Sa Ngam.
Dont worry about your passport, I have been refused entry into Vietnam four times trying to get in with my bike all times I had been stamed out. Three from Cambodia and one from Laos. I have never had a immigration officer ask me any questions about this when travelling anywhere else in the world.
I also have a Thai registered bike and have all the paper work.
The Thai's had no problem with letting me go out or coming back in, it was the Kmers who would not allow me in at Chong Sa Ngam.
I got in at Osmach but don't know why or if I had been refused - why not. The only paperwork was done by the Thais and the Kmers just put their stamp on it. Communication is not a strong point with these guys. My impression is that they try to slow it down as much as possible. I don't know if they were looking for some payment but they didn't ask. It may also be that it is just too hard and they don't know what to do. It probably doesn't help that border disputes are going on in this area at the moment also.