Right, I'm getting portal soon, but for those who have it and cant get enough:

The portal flash game (and yes, it is addictive )
http://portal.wecreatestuff.com/

And how to get portals in HL2:
http://www.primotechnology.com/2007/10/17/half-life-2-portal/

I played the flash portal version, got so far and then got to a level that looked impossible (had to make a jump, but with loads of walls in the way and no way to place portals).

As for the playing of HL2 using portals, that looks pretty fun, though it's basically just copying all the maps across, lol. I may try that at one point.