here my three cents/stories on crossing with a (Thai) vehicle from Thailand into Cambodia (Poiphet)
1.) years ago (2003/2004) I was working in Bangkok and brought my Malaysia registered off-road truck to Thailand. At that time you had to leave Thailand once a month with your vehicle, so I drove on a monthly base to Cambodia and back, without any problems (was even able to fill in the Thai forms myself after a while). But these days are gone after some political hooligans in Thailand brought up the topic around Preah Vehear temple some years ago and after a regional mini war, the Cambodian site brushed up their national pride and stopped allowing any Thai vehicle from entering Cambodia. (But even earlier I wittnesed some strange stories: I met people with a Malaysian registerd truck at the Angkor Wat complex, who had been 'forced' (for their security) to accept a police motorcade and the car had to be driven by a Khmer police officer throughout the trip, obviosly with some hefty service fees attached. The reason for the police was, that left hand driven cars are not permitted in Cambodia. What a BS story, still today (2013) plenty of left hand driven cars are roaming the national route between Siem Reap and Poipet as private taxies often dangerously overloaded. How many Khmers fit in a car if you want to keep the fare low? You can't beleave it.)
2.) in 2008 I flew to Phnom Penh, only to get to the Custom HQ to get a temporary entry permit for my then Thai registered car, as we transported donations for an orphanage to Batambong. In fact the officer in charge was really friendly and helpful but also reminded me we should wash all the cloth before as many deseases could come with them (especially from Thailand). However, as we popped up at the Khmer side custom office to get the car in, all red tape broke loose as the officer in charge at Poipet felt not authorised to accept the entry permit from Phnom Penh HQ and we should get the needed additional authorisation from the district custom office in Sisophon, some 30 km further inland. We finally got away with our car, as I was able to pull out a Khmer driving license (expired and only for Motorbikes) and everybody was so surprised that they gave up and chased us out of the office. We even stopped in Sisophon at the custom office, got the additional what ever blessing and off we went. But I guess we had been the only Thai registered car at that time in Cambodia. I got a second temp import license again from the same officer in 2010, however, my wife was not interested to test my negotiation skills again and I never used it.
I must positively note, money was never in discussion (or in question), over the table nor under the table.
3.)
two weeks ago (Oct 2013) I drove with my baby BMW motorcycle over the border at Poipet and faced absolute no problems. The Thai site was very smooth and professional (with paper work and stamping 30 minutes max, but also reminded me that I might face some issues on the Khmer side with Thai registered vehicles) and on the Khmer side visa and Immigration (15 min). On the way back, I stamped out my passport at immigration (5 minutes) drove over to the Thai side, first vehicle immigration stop (3 minutes), Thai imigration entry stamp (5 minutes), custom stop (3 minutes), all together less then 20 minutes.
I will cross the border at Poipet again end of November for a longer off-road trip and with all document copied and sorted it should be even faster.
Never went over any other border stations but would like to get over with my bike at Ko Kong one day. Will be interesting to read if anybody has expiriences about that.