Oh Blizzard why do you torture us so? Last time it was all about Gilneas, and this time, Wyrxian winks at a player suggestion that phasing gets used more in the old world. Take this all as you will -- Blizzard is just pulling our chain on this stuff, and whatever they have in mind, they're not giving away quite yet.
But, let's speculate a little, shall we? We already know that there's one more Lich King mystery content patch: Ulduar is coming first in patch 3.1, and Icecrown will likely be the final raid of the expansion, but there's one more raid that they haven't told us about yet. So we could guess that we'll finally see what's been going on in Gilneas all these years, and that it'll be a phased experience, much like what we saw in the Sunwell patch before the expansion. Plus, the announcement for that patch could be right around now -- with 3.1 set to go live any day now, Blizzard could release that patch and then throw out a preview for the next release right away.
That's all guessing, of course. And we have no idea whether the phasing reference and Gilneas are even in the same patch -- I speculated long ago that the Emerald Dream expansion could be an entirely phased version of old Azeroth, which could completely revamp the 1-60 experience. So who knows? But Blizzard, as they always do, know something, and while they might let on soon, they're not telling right now.