Finally, this weekend, I got a postcard from SanDisk saying my rebate had been denied because the purchase was made outside the allowed date range. I checked my photocopies of what I mailed in, and the offer was valid on purchases from 5/9 through 5/22. I'd made the purchase the evening of 5/22. Amazon's packing slip, though, had showed a date of 5/23, presumably when it was packed and shipped, and that must be what SanDisk had looked at.
So I e-mailed Amazon to ask them to convince SanDisk to honour the rebate, or credit me the $13 themselves. An hour and a half later, I got an apologetic reply saying that they had credited my credit card the $13 immediately.
Thanks, Amazon! This is why I keep buying stuff from you.