There is a tradeoff here -- if your ticks come faster, then the HoT or DoT component will end earlier, which means you'll be casting the spell more often. GC isn't sure (and neither are we) about how that will balance out yet -- on the one hand, it means that within a certain period of time, you will be able to do more damage or healing... but then again, you'll be spending more mana to do it. So instead of throwing this into talents, they're starting it up with glyphs. In the next patch, priests would get three glyphs that make their instant cast spells have their DoT components affected by Haste (Mind Flay's glyph is likely being moved into the base spell, and they'll use that one as an extra glyph choice). GC says if it works there, it may be spread out into talents or even base spells.
Will it work? It's an interesting idea, but mana cost seems to be the toughest problem to deal with -- any time you're casting spells more often, you'll be going through more mana. But it does seem like a good way to make Haste a little more worthwhile, and especially with the stats refinement coming up in Cataclysm, Blizzard will have to even out all of the stats they can. What do you think?