In the case of Balsamiq, I think it would be pretty simple. My files are saved in BMML, which is just a simple XML dialect. I suspect someone with good XSLT/HTML/CSS knowledge could write a little BMML->HTML+CSS script in a couple of days.
That's one of my ideas for future startups. In fact Balsamiq is very close to it: the output is xml.
Question to you or anyone else on this thread - are there any tools out there that could convert a UI mockup for a webpage (like one created with Dabbleboard or Basalmiq) into standards compliant html/css?