Carrier unlocking and Rooting are 2 different things.
Unlocking will allow you to use the phone on other networks (Bell, Rogers, Fido, etc...). This will NOT allow you to flash other ROM's like FroYo being released in the UK soon.
Rooting will give you access to the phone so you can load up any ROM designed for the phone. Rooting will NOT unlock the phone from Telus.
2 different process to achieve the results you are looking for. At least the rooting portion is free (see link below). It is not known at this time if the phone can be rooted using this process. When the Telus HTC Desire actually gets released ?tomorrow?, we'll know for sure if this method still works. I don't see any reason why it wouldn't though.
http://unrevoked.com/
HTC Sensation - CM10
HTC Desire - *SOLD*
HTC Magic - CyanogenMod 6.1.0
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