Well, after finding the drive Out of Stock at the website yesterday, I printed out the website page, and called my local store. They had the drive in stock. So, I went there, and asked them if they could do the deal the same way. I told them the website had an online coupon, as well, which I definitely wanted to use. They said we could do it all. After they rang it up (with $30 coupon), they were stunned when the rebate coupon didn't automatically print out, so they printed out the online mail-in rebate from their PC, from the website. They assured me it would be honored. I called the Office of the President, and the CSR assured me that if it wasn't honored by Maxtor, to call back and she would honor it (of course, I have her name and extension...). Hope that works!
<Edit> I just got off the phone with RebatesHQ, speaking to a CSR. He told me that the MIR is good for the retail store, also! He gave me his operator number and name. I asked for some kind of email or validation in writing, and he gave me an email address to write with my request. If I get a response back, I'll post it here. So, we may be able to buy at the store, use the $30 off $100 coupon code (see my earlier note on where to look), and get the $20 MIR as well!