I got the one in Canada. Absolutely no problems at the border.
I did a little online research and learned that trailers are duty-free, but required to meet US FMVSS regulations. As long as the trailer has the compliance tag, the process is easy--worst case you have to fill out a form. If it doesn't have the compliance tag, then you need to use a "Registered Importer" which sounds like a right pain.
I checked to make sure the trailer had the required tag (it did). At the border, I declared it (even though I most likely could have gotten away with not saying anything). The CBP guy took a quick look and sent me on my way--no hassles with any form at all!
Phil, I owe you breakfast at the next meeting! Or if you're coming to the end of year banquet on Saturday, I'll buy you a drink instead