The Canons both blow the D90 away in video.
The 7D has a metal, weather-sealed body.
The D90 will likely be replaced by a new camera very soon.
The t2i is cheaper than the 7D and the D90.
The t2i is less than a year old, the 7D is about a year old, and the D90 is over two years old.
Canon high quality lenses are generally a little cheaper than Nikon ones.
The t2i has the same sensor and metering as the 7D which means that you can technically produce the same high quality images with either camera.
the t2i and 7D's sensor performs circles around the D90 in low light.
All three cameras have live view and video, the Canons have allow fully manual control over video exposure as well as many different frame rates and resolution settings. The Nikon's video is automatic exposure, no resolution or frame rate options and only half the resolution of the Canons (Nikon = 720p, Canon = 720p and 1080p)
The Nikon has more focus points than the t2i, but the same as the 7D.
The D90's focus points are more spread out than both of the Canons but only has one cross-type focus point. The 7D has 19 cross-type points.
The D90 is an awesome camera. I used the hell out of it until I bought my 7D.
The 7D is a much better camera than the D90, but costs twice as much.
The t2i shares the 7D's sensor, but is in a more simplified body and costs half as much as the 7D.
I think the D90 is built a little more solidly than the t2i.
I own a 7D and will likely be buying a t2i to use as a backup body in the next couple months.
The D90 is great and was, what I thought to be, the best camera by far at it's price level, but I think the t2i is a real knock out at $800. The D90 is likely to be replaced in the Nikon lineup before the end of the year, so you may experience a little buyer's remorse if you drop the coin on one now, only for it to be upgraded in October or something.