Les, what version of UVMapper did you use? In my experience, both Pro and Classic start the output file with the comment:

file generated by UVMapper

• then follows some statistics on the mesh. The comments are never in the middle.

My highest installed Poser version is currently 7, and it's running a render at present, so I can't test - but I do know Poser can be picky about OBJ file formatting.