i just crossed the Poipet border into Cambodia this saturday (on my german registered honda transalp),
as i gonna stay in thailand for longer i had to get my bike out.
So this crossing was very important for me and i was a bit anxious about the fact that i overstayed my temporary import permit for Thailand and had to make sure i don't get a "import registration" from Cambodian customs (i want to park the bike there for 2 years).
well, everything worked like a charm, arrived at the border at 2pm, queued up for thai immigraiton exit stamp, went back to get my bike, got stopped by Thai customs to hand over my temporary import permit, paid 1000 bath (i overstayed 10 days, you pay 100bath for each day you overstay, but a maximum of 1000bath if you overstay less than 2 month, a maximum of 2000bath if you overstay between 2 and 6 month),
continued to the Cambodian side, got a visa, queued up for immigration, went back to the bike and just went trough where all the small scooters where going through (no gate).
The Cambodian customs office is just a few meters from where you queue up for immigration and i parked my bike 4 meters away from the office. at some point some officials where "playing" with the bike, but when i finished my visa nobody was there.
So i think it's kind of a voluntary thing to do customs in cambodia
Beside all that the border was so crowded with tourists that it took me 4 hours to cross...
I heard stories of people that have been turned back because they had no carnet, but i believe that was because they tried to enter with thai rented bikes, which is a totally different story.
Hope that's helpful for someone,