There's a chain system in place for system updates. Android devs release the update, phone manufacturers tweak the update (e.g. HTC tweak it with HTC Sense UI), then the phone carrier (e.g. T-mobile) tweak it with their own crapware. Once all of that is done, a few months have passed and you might get an OtA update on your phone.
My HTC Desire HD is on 2.2.1. I know 2.2.3 exists and there are news articles about 2.3 being promised to us, but I've still not got the update. 2.4 and 3.0 are also out, but they're being aimed for tablets with Tegra 2 graphics.
So far I've not heard of any phone being developed with Tegra 2 graphics, although there are phones that will have 1.2Ghz dual-core Scorpion CPUs rather than the 1Ghz Snapdragon CPUs that all high-end smartphones currently have.