Do what I did - call that 888 number at the bottom of the WD Rebate form. I told them I was thinking of ordering a hard drive that is backordered. If the hard drive were to be backordered for say 28 days, would they base the "must be mailed 30 days after" receipt thingy upon the date I received my drive or the date on the receipt. I was told that even if I hadn't received the drive yet, I should mail in the rebate form. They would then send a letter later saying "hey you forgot your UPC" basically, and to include it then.
So do what I did, be sure to write down the persons name that you spoke to, include a note inside your rebate form envelope explaining the situation, write down the date and time you called, keep copies of everything you send them, get proof of delivery when you send the rebate. This way if they give you any crap about the rebate you can tell them you talked to so-and-so on this day, and this is what they told me, and you know they received the rebate form on XX day.
Good luck.