Thought I'd start a discussion on this since it just happened to me.
Installed a standard Leviton 7599 on a 20-amp circuit in my kitchen last spring (2010). It's an isolated device; it doesn't protect anything downstream, and everything up- and downstream works fine, so it isn't the wiring. GFCI will not reset, indicator light is dead. I think it just failed. Haven't replaced it yet because (a) we rarely use it and (b) I'm lazy.
I've installed five of these so far, for a 20% failure rate. Since they're made in China, I assume they're prone to failure. Beyond the inconvenience of needing to replace it, the failure really makes me wonder about the effectiveness of the GFCI protection itself.