HTC Desire: why not to root and install Froyo (no-porting)? I've bought an HTC Desire a quite ago and, since now, works fine for me (except for this).

Now I'm waiting for Android 2.3, or 3.0, or whatever, and I'm thinking: is it possible to install the original Android ROM on my Desire instead of HTC porting?

This will make spare me a lot of time in waiting HTC porting.
My only doubt is if all functionality stay the same. (except for Sense and similar HTC proprietary software).
There are many ROMs available for the Desire based on stock Android / the open souce project. I currently run Cyanogen's ROM on my Desire, but I have also used OpenDesire and many others.

Take a look on the XDA forums.

There is no 'Official' stock ROM for the Desire, because no one is selling in without the HTC ROM, but OpenDesire is as close to the open source project as you are likely to get, where as Cyanogen's ROM is more feature packed!

You will of course need to root your phone to install custom ROMs. This is easily done on the Desire using Unrevoked.