Joyant, the people behind TextDrive and Strongspace are really keen on OpenSolaris and ZFS and actually use it in their production environment, and they're not afraid to advertise it either...

I don't use any Joyent products yet (but might when my current hosting plan runs out), but I must get myself one of these T-Shirts (I prefer the plain one).

Update: It's been a while since I wrote this post, but thanks to Jonathan Schwartz's recent post - Technology Adoption (2 of 4) - this post has suddenly become a lot more popular. I can confirm I am now running ALL of my sites on Joyent on OpenSolaris and I can say I'm very very impressed with the performance and reliability.