YouTube - Portal - Challenge 13 - Least Steps - 9 steps
Youtube is your friend. I've also seen it done in 5 if you want to feel totally inadequate like I do.
Necronopticous: Here's challenge 13 done in 2. YouTube - Portal challenge chamber 13 using only 2 portals
Heavy use of some exploits, but damn. The fact that people were figuring this out within two days of release is impressive.