We're looking to update our Asterisk installation, and was wondering what most people consider "stable" at the moment. I see that 1.6.0 has been released, but I don't think going to a .0 of anything is a good idea (I may be wrong). We're currently running 1.4.18 with 2 T1s and about 120 SIP phones.

Pretty much all the Asterisk 1.4 versions after 1.4.18 are stable. We are currently running 1.4.21.2 on our new installations.

On the Asterisk 1.2 branch pretty much any version after about 1.2.19+ is stable.

I'd assume 1.4.22 then would be good (it's the latest in the 1.4 branch). Any recommendations on wanpipe or zaptel/dahdi, or will the latest be fine?

BTW, I'm assuming that I can't use the Zap channel prefix anymore on current DAHDI releases? Time to update the dialplan, I think..

1.4.22 would be fine.

Latest version of Zaptel and Sangoma Wanpipe is fine. LibPRI as well if you are using PRI. These are almost always fairly stable.