Updated Steripens?

Actually, I just returned mine for the exact same symtoms. Rich, the service person, said it sounded like a bad bulb. The tech sent back a scribbled note stating "circuit problem." The old batteries that I was using, did indeed work when it came back. I tested it with three sets of batteries: the old set, and, 2 new sets.
All did the same thing you describe before the repair. BTW they sent me a new one.

Thanks everyone for the very quick info. I've also emailed Steripen, and am waiting to hear from them.

The batteries were Energizer lithium CR123's stored at room temp (55-85 deg. over the course of the year, probably) in their own Ziploc. (They were the original batteries that came with the unit.)