I recently had this done at the dealer, and they listed all the things they did to replace the part. I don't remember all of them, but they did have to remove the upper left engine mount! So I'd say its a pretty big job. I took a look after they did the service, and the clearance between the tensionner and the body looks really tight, so that could present even more problems.
If you do decide to do it yourself, remember to prime the tensionner before installing the belt (the procedure is shown in a TSB you can get from the links above.)
i've done a few of those tensioners. they aren't fun. you do have to remove the motor mount like marcus mentioned, then you have to jack up the motor to make enough clearence to get the bolt out. i'd problably take it to a dealership if its not too expensive. just don't take it to mine because i don't wanna do it
I just had mine done yesterday. I have 60K miles, but luckily my warrantydirect.com extended warranty covered it all except for the belt. It would have costed about $350, but instead just had to pay for the belt which was $60.
The guy at the dealership who worked on it, is a good friend of mine. Been friends with him before I ever had the Celi. He was cursing me out when I came to pick it up. He said he's done it on the GT before, but this was a first on a GTS. Toyota only rates the labor at .6, but yet it took him about 2 1/2 hours, he said it was a major pain in the ass, because there is absolutely no room to get it at. With the GT he said it's much much easier. Yes he also mentioned he had to unbolt the engine and jack it up. All I can say is have fun.