All the common statements, are that the betta would pick on the guppies. I had the opposite problem! I had a 10 gallon tank, with 3 male guppies (no females, since at the time I didn't know that was much better for alleviating stress), and a betta, along with a couple mollies and neons.
Well... my guppies ganged up, and chased/nipped the betta! So now he's in a 2 gallon tank at work.
(and if you're wondering why my sig. doesn't have guppies, mollies or neons... they got Ick and died >_< )