Hi Dan,
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.
Cheers,
Peter