Someone mentioned that problem before. I've never been bothered by it but then my terrain textures are usually fairly random stuff so the effect isn't very noticeable. There should be a simple fix. I'll post back later when I've had a chance to do some tests.
Is it just the main texture or the detail texture as well?
Edit Just done a quick test and it seems that the terrain's size is slightly smaller than it should be and the UV coordinates are off too as you say. The amounts probably depend on your heightmap dimensions which is unhelpful. The only real fix is to recalculate the terrain's position and UV coordinates yourself - or just live with it. This slight unpredictability of Advanced Terrain dimensions is one reason many people use their own terrains or one of the other terrain plugins. It's a shame really because it was a good idea marred by unnecessarily sloppy implementation.