Mirror's Edge has now been released in the U.S. and is now available on Steam for US Residents, this only means that it is days away from release here in the UK.
The best pre-order deal I could find for the game is at softUK for £21.24
To get the deal you must be a SoftUK Collector Account Holder where you get free delivery. The account is free to open.
Details can be found at http://www.softuk.com/Softuk_Collector/index.asp
If you decide to buy the game when it is released and they have it in stock, I believe with the attached voucher you may be able to get a further 10% off, but please read the t&c of the attached voucher first please.