Last time I tried to enter Laos via the ferry to Thaket I was rejected on Laos side. They wanted to see a "Vehicle Registration Certificate" besides the green book. This paper is issued at any "Department of Land Transport" (where you get your driver's license and can pay the bike's annual tax) but it is best to go where you have your bike registered to avoid extra nagging from the staff. The paper costs 25 bath. Bring your "green book", passport and signed copies of your passports first page and VISA stamps. Two friends of mine got across the friendship bridge in Nongkhai into Laos with this paper. I recommend anyone going to Laos to get one of these papers just in case.