That's their average time. The actual fail rate likely varies one or two years.
And as for the fans, I'm not talking about their dbA ratings, I'm just talking subjectively. At 2000RPM my Panaflo H1 is quieter to my ears that my Ultra Kaze 2000. Performance is pretty much the same though.
A fan's MTBF is something to factor in, especially for people who keep their PCs on 24/7, but I have yet to see anyone reap the benefits of keeping a fan for 5-10 years. It's something to think about when purchasing, but I don't think you should base a purchase on the "MTBF alone".
Edit: Peace of mind from your other post can certainly be weighed in.
Edited by sunburn39 - 7/6/08 at 5:05pm