Mail in rebates are effectively you loaning the company you are buying the equipment from the rebate money.
So all of you buying a 6D or 5DIII and looking forward to that $400 extra, well what you've done is said "Here, B&H, you can have $400 of mine for free!"
Mail-in rebates typically have a specific window of time during which you must remember to mail in the barcodes (or whatever) or you don't get your rebate. Then it can take a year or more to receive the rebate.
From the time you pull the trigger until the time you get the rebate, you have effectively loaned B&H $400 for free.
Mail in rebates are a scam and should be avoided like the plague.