Boy, we've run through the gamut of opinions here - from accountants to finance professors, marketing weenies, and economics profs. Great question.
I side primarily with the marketing and accounting views. First and foremost, the tax laws as was astutely pointed out earlier. This may be over-simplified and not spot-on, but from what I recall in classes 20 years ago, the companies get to "set aside" a huge wad of dough, as if all avaiable rebates would be turned in, and this sits around earning interest. Also, this large amount of money is off the books for tax purposes, decreasing the amount they are liable to pay taxes on for that quarter the rebate is good for. And of course, they ALWAYS have rebates going on.
The other reason is marketing - perception of a good deal and a normal higher price. If the price was always $10, you'd then expect someone to have it on sale for sometime for $9. If you have it at $39 with a $20 rebate, you can still sell many at the $39 in-between rebate times, sell many at $39 for those that forget or lose rebates or miss the deadlines, and sell many many more at the true value, perhaps, of $10.