The reason we likely haven't seen it much yet is because so far, spell haste stuff only drops at the highest levels, and, from what I can see, in the form of temporary buffs that allow you to get a few casts off quickly. An equipped high spell haste rating would fundamentally change the way the game was played in many situations-- if mages could kick out a quicker pyroblast whenever they wanted, that would have a big effect in a lot of PvP situations.
Tseric also clarifies that this only affects cast time, not DoT tick time, or duration of spells. And while the two trinkets linked above are the only places I could find it in the game so far, I wouldn't be surprised if this rating found its way into certain instance specific buffs, either for us (players could get a DoT put on them that would kill them in 30 seconds, but let them cast all their spells almost instantly within that time) or against us (mind-controlled players get their spells hasted for a short period against the raid). Either way, as more and more players move into the endgame, spell haste will be something to look out for.
Update: Commenters have found a few other spell haste items, including this 60 trinket, and an awesome BoE caster sword.