Amazon is now offering Super Street Fighter IV with a $10 Amazon credit, the bonus pre-order costume pack, AND release date delivery for $45.94 (if Amazon doesn't charge tax in your state).  They originally just had the costume pack, then they matched Walmart's $10 store credit offer, and just today (04-08-10) they've finally made the game available for release date delivery.  Easily the best deal I've found.  
 
If you don't need to have the game day one, you could also wait a week for delivery and just pay the 39.99 retail price.  The costume pack and $10 Amazon credit are for the pre-order, and do not depend on selecting release date delivery.
 
[edit]  Almost forgot: the above info is for the 360 version of the game.  Amazon is selling the PS3 version for 3 dollars less, and I think all the deals listed above still apply.

Seeing as the game will be out in a little over a week, I thought I'd bump this.  It's still the best deal I've found, but if anyone knows of a better pre-order deal, I'd love to hear about it.

For England, the best price for preorder is £19.29 at Tesco Entertainment. To get the 15% off, all you have to do is use promotional code FTSL15-1.
 
However, it is £21.86 at shopto.net, with a free Dhalsim T-shirt for preordering.

" For England, the best price for preorder is £19.29 at Tesco Entertainment. To get the 15% off, all you have to do is use promotional code FTSL15-1.  However, it is £21.86 at shopto.net, with a free Dhalsim T-shirt for preordering. "

" @dbz1995 said:

" For England, the best price for preorder is £19.29 at Tesco Entertainment. To get the 15% off, all you have to do is use promotional code FTSL15-1.  However, it is £21.86 at shopto.net, with a free Dhalsim T-shirt for preordering. "

Yeah I paid the extra couple of squids for the cool shirt. "

" @SpaceInsomniac: Did you get your bonus code yet? "

lol, i got my copy today from amazon. it only cost me $25 through deals and past credits

" @SpaceInsomniac:  I got my costume code in my e-mail this morning. Perhaps you've got yours too! "

Yeah, i got a $4 rebate.  Made me regret being such a cheapskate and not getting the release day shipping.  Got my costume code too...damn.

Perhaps a dumb question here, but are the costumes from the amazon bonus the same two costumes you get from having a SF4 save on your system?  I downloaded the costume pack using the code I got via email today and don't see any new costume selections.

It's only costumes for 4 or 5 characters.  The things you get for having a SF4 save is colors 11 and 12.  A stylized look for your character.

Ah, I see them now.  Dhalsim, Gen, Guile, Blanka, and Fei Long.   
 
They should have thrown in THawk and Dee Jay to round out a solid group of characters I very rarely use...