Frostfire builds are still very viable and are actually better at certain gear levels than arcane-heavy builds, at least for PvE. Personally, I don't raid on my mage very often and just don't have the gear to support an arcane build. I get much more mileage out of both my stats and mana pool size with a frostfire spec. I've tested my FFB spec against the cookie-cutter arcane specs with my current crappy gear; while my "damage per second" was higher, it wasn't sustainable for longer fights. If I had full Naxx gear or better, I could see making the change to fire/arcane, but I would actually reduce my overall damage output by doing so now (confirmed after much testing and spreadsheet work).
PvP-wise, it's still a toss-up between frost and arcane for me. Frost just has so many survivability mechanisms and so much utility that it's hard to pass up some of the deeper talents. I guess it really depends on what you like to do, though. I'm more of a battleground PvP'er (I don't like the Arena system much) and having ice barrier, the water elemental for the frost novas, and a blizzard that snares just come in too handy in places like AV or EotS
Ultimately, I love that top-end mages are doing so well with arcane...I'm just not geared nearly as well as they are (
) and that's something to consider when recommending specs. What I do like most, though, is that we finally have a few spec choices that are viable instead of only one clear-cut, hands-down winner.
Oh, side note...any rogue that lets a mage dictate the course of the fight is a free kill to begin with