When it comes to strings for fretless, I hear the greatest things about Thomastik Infeld (aka TI) flats. They seem like a low tension (easy to play), great sounding (like a double bass, which is waht fretless should sound like), and extremely long lasting string. I wish I spoke from experience, but as soon as I get my fretless, you better beleive I'm dropping $50 on a set of TI's. I can't wait to try 'em. Check out this thread I started:
Flatwounds for Fretless...
edit: The other flats I've tried on fretless is D'addario Chromes. They are nothing special, and the string tension is too high to play comfortably for an hour or so. This comes into play with fretless because it's harder to finger notes on a fretless. The chromes are great for my fretted acoustic-electric, though, because like all flats, they reduce fret buzz to almost zero and feel greet on the fingers.