No problem these days, The bikes on trucks scam has been eliminated, you can ride across. Thai customs and immigration at Nong Khai is computerized, (ie: quick) and many "Falang" on "Big Bikes" are crossing into Laos, so Laos customs are much better at processing them.
Ulrike and Kai are right, it is cheaper to get your visa in Bangkok.
For up to date riding info check out gt-rider.com There is a laos section that will answer any of your questions, and David Unkovitch has good Laos maps available.
Enjoy the Beerlaos...
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