In December 2013 I rode my 2008 ktm 690 into Cambodia from Thailand in Piopet. They were holding 2 guys from Singapore at the border because they did not have "correct import paperwork".
We went into the customs office and they tried to tell us that we could not enter with out the customs documents.
Our reply was "well, give us the documents and we will fill them out".
"You had to get them from our government before you come into our country".
We said "no, this is not how it works." And I proceeded to show them customs documents from Russia, Mongolia, Thailand, and Japan. After about 20 minutes they photo copied our registration and put a stamp on it with the date and that border crossing. They didn't keep a copy for themselves. We didn't have to pay anything.
I left the bike in PP for a few months while I went home to see family and went into vietnam. I left Cambodia after about 3 months into Laos. The customs guy gave me no problems with documents at the border. Laos was even easier.
They generally don't know whats going on. So you can tell them how it normally works. Unless you get a guy who needs to give you trouble that day. then you might have to hand over paperwork ($$$)