We shipped our motorbike from Kathmandu to Bangkok on May 2013. And we didnt faced with any difficulty to take our bike from the custom of airport. We didnt need any agency to do it for us, we did all the process by ourselves and just paid aprox. 1300 baht to the warehouse for 1 night.
İf your bike hasnt been shipped yet, it is better to ship it on week days except friday. At weekend you cant take your bike from the custom, and you have to pay for 2 nights.
As Keith remind you, in Thailand you dont need carnet. Only documment you need is shippment document that the agency (in Tokyo) gave you. Most probably the agency shipp the bike by Thai Cargo.
Just go to the custom early in the morning. beacuse it takes 2-3 hours to complete the paper works and may be 2 more hours to remontage your bike.
For entering custom area, you need to get an enterance card for 50baht (as i remember )
Dont worry it is not so hard, warehouse officer and workers are so friendly and helpfull, dont worry about it.
good luck, all the best.
if you need more information our e-mail adress is : firstname.lastname@example.org
Hakan & Sandan