Thanks wifiguru, I kinda hope you're right. Hmm...what about these thoughts, which are why i think it's software and not hardware:
1) This problem started *exactly* when I installed Yosemite in late October 2014. It seems like too much of a coincidence that both events happened at the same time: Yosemite install and wifi card dies?
2) Apple wifi engineering has contacted me multiple times through the Apple bug reporter website, asked for many wifi diag logs, and even called me on the phone in February to collect data on this problem. They didn't mention the possibility of a hardware issue. If they thought this was likely, I think they would've closed my bug with the note "go to an Apple store" like they did with my (unrelated) iPod Touch problem.
Apparently Apple wifi engineering didn't see a hardware problem in the numerous wifi diag zip files i sent them (???).
3) A wifi card shouldn't die after only two years of usage.
4) Apparently a lot of other people have Yosemite wifi problems, ever since the first release, so my problem seems to fit with theirs.
5) I was on the phone with apple tech support in December 2014 and they didn't mention and wifi diag i could run that would point to a hardware problem.
6) the problem is so intermittent, I'm not even sure i could reproduce it at an Apple store, given the advanced scheduling required at the Genius bar.
BTW, i rebooted to Safe mode, and that brought wifi back. then rebooted again into regular mode and wifi has been up since then.
regards,
js.