Can I unlock my 3GS with OS4 & baseband 05.13.04?

[rupchurc]

My question is simply this. Can I unlock my 3GS with OS4 & baseband 05.13.04?

This website seems to think so but I wanted to double check and see if anyone knows for sure before I go flashing my phone and such.
Unlock iOS 4.0 iPhone 3G/3GS Baseband 05.13.04, 05.12.01 with Ultrasn0w 0.93

My other question is, once unlocked, If I keep my AT&T sim card in can I still use AT&T? Will I only need to swap in say a T-mobile sim card and the service would start working? Reason I ask is Im selling the phone to my friend who wants to use it on T-mobile and I told him I would unlock it for him, but I want to make sure when I send it to him all he has to do is put his T-mobile sim card in and be good to go.

Any feedback appreciated!

[6309phone]

3GS new bootrom 3.1.3-> 4.0