I live in Surin, so the closest border crossing is O Smach, which I used often when I had the XR650, but only in the dry season. Now I ride a Ducati Multistrada 1000, the perfect bike for Thailand, but not a dirt bike. So I tried the Chong Sa Ngam/Anlong Veng crossing 130 km further East. Early December, the checkpoint was almost completely shut down for lack of traffic, due to the still-simmering border tensions. The Thais were extremely friendly (not computerized), I signed off on my 3-mo. temporary import permit. Then an emissary from the Cambodian side came to inform me that the bike would not be allowed in. I went over to the Cambodian customs shack to negotiate and was politely but firmly rebuffed. I asked them to check with their superiors, which they did, and their position was confirmed. I needed to obtain permission from customs in Phnom Penh prior to admission with a motor vehicle at Anlong Veng, even a motorbike. I speak fluent Khmer, and I argued with them long and hard, even calling a friendly customs official in Samrong who confirmed that in his province (Oddar Meanchey) there wouldn't be a problem, but that Anlung Veng was a different jurisdiciton. I had him speak to his colleagues, but it was no deal. Must get permit from Phnom Penh. As a last resort I told them that I thought this was just a scam to squeeze some money under the table, which was no problem for me as I was a rich foreigner. At this, they got very offended and even menacing, so I retreated to Thailand. Got a new temporary import permit valid 30 days.
Next day, I went to Poipet, via Aran. There, the senior Thai customs officer was very rude, cursing bloody foreigners who evaded import duties by using the "loop hole" afforded by the damned temp. imp.permits, and blustering that this hole should really be nailed shut. But on the Cambodian side, nobody at customs even glanced at the bike, conspicuous as it is. No paper work aside from the visa.
Then, the worst stretch of road in all of Cambodia began, just after the roundabout. It had rained there that morning. For about 300m, there were trailer-trucks bumper-to-bumper, and every space in between was so jammed with motorbikes and the odd Camry or Land Cruiser that nobody could move. Grid lock on main street--now a 2-way track made of large broken rocks the size of basket balls, covered in 30 cm of very slick mud. It took me half an hour and 2 liters of sweat to make it through to the other end, exhausted but safe.
From there to Phnom Penh took all of 4 hours, the road via Battambang/Pursat was in the best condition I had seen it in in 15 years, or ever.
I went straight to the customs office to ask for a permit to enter by motorbike through Anlong Veng. I got it within 24 hours, at no charge and valid until end of March, although they're usually issued only 'till the end of the current year, I was told. Permit is renewable, valid only for this bike, actually consists of my annotated and stamped request, with chassis & engine nos. noted, photocopies of ppt & vehicle registration. It took me 4 trips to the computer shop to get the letter just right, proper typing, proper heading/form of address, proper information, proper length (must leave at least 1/3 of the page blank for comments and stamps, so cut out the crap about your sick mother-in-law and visits to your development projects).
The road from PP to Anlong Veng is about 430 km, branch off from the highway to Siem Reap. The new road is clearly visible on satellite images, but is not yet included in the road map overlays on MS Virtual Earth, for example. And believe me, this road is spectacular! A 4-lane highway through the jungle and hills, totally flat surface, mostly tarred at the time of this post, and practically no traffic at all. You could do 200+ km/h on some inclines with 2-3 km visibility. And seeing that, I realized that the requirement to obtain a permit in PP actually makes sense, as it would be extremely misleading to enter Cambodia on such a road only to then hit the usual Cambo roads elsewhere unprepared. Believe me, friends, Cambodian roads are still dangerous, Cambodian traffic is still deadly! I have experienced over a dozen road accidents in that country over the years, one of which ended up costing me my spleen. I paid my dues, I have the scars to show I earned my way to freedom. Oh and how sweet it is now!
I returned to Surin (560 km from PP to my home) in 6 hours. Everyone on both sides of the border extremely cordial. When I went back through there again a week later, the Cambodians made a photocopy of the permit, stamped that, and asked for it back when I left. I get to keep the original.