The Sparkfun guys and we diagnosed the issue today. It was our fault--we had a version control problem with the Attiny code we sent them, and the bad news is that appears to have affected a total of 60-80 boards. For those of you affected, our apologies--we'll make this right. Here are your options:
1) The fastest way, if you have an AVR programmer, is just to reflash the chip with the updated code (hex file is here; full source code is here). Instructions are here. If you don't have an AVR programmer, but have always wanted one, this is your chance! The AVRISP2 is excellent, comes with the terrific AVR Studio software, and is cheap ($34)
2) If you don't have an AVR programmer, the solution is to return the boards to Sparkfun and they will reflash or replace them at no charge, testing to ensure that you have a working board. Just send an email to firstname.lastname@example.org, who have been alerted and will help you with the process.
All board shipped from today on will have the correct firmware, so this should be the end of the problem. Again, our apologies for the inconvenience and confusion. Our aim is to ensure that everyone has a working board as soon as possible, and Sparkfun has been great about responding quickly and making sure this doesn't happen again. Thanks for your support and understanding.
[Update. If you have this problem and live in the LA area, Jordi has volunteered to come to your house and reflash the board himself, no charge. Just PM him. For those in the Bay Area, I'll do the same, although depending on where you live, we might have to meet at some mutually convenient location. For those in Europe, we might be able to match you with someone who lives near you and has an AVR programmer. Please post in the comments if you need that.]