I purchased a 600 last year off of ebay, it now has its 2nd SIM card in it.
All I had to do was put it in, turn it on and it automatically detected the card and registered the phone on the network. That is the beauty of it being unlocked, no dealing with mobile phone companies.
If you are serious that this is the unit you want, I would be willing to sell you mine (I found a new toy I want to spend my $$ on LOL)