I pre-ordered the CE edition of GT5 around September. I can't find any transaction on my statement that proves I did so. I ordered it from gamestop.
 
So my question is: should they have already billed me, or do they bill me on release day?

They charge you for the game when you go pick it up, if that's what you're asking.

...did you use cash?

How did you pay for it then?  You are going to need something to pick it up but I would be surprised if they had charged you for the full amount when you preordered it.
 
It is absolutely amazing by the way, get on it hard.  If you still have a preorder that is!

Last time i was there it was $5 down to pre-order and pay the rest whenever. I have only ever used cash, but they should have given you a receipt and asked you for your name and phone number in case you lost the receipt.

  @Lego_My_Eggo:@Mrskidders:@Devil240Z:@FancySoapsMan: 
 
I pre-ordered online, as in it will be shipped to me. So do they bill me when they send it?

Oh... Online. Missed that part.