I've just returned from the Apple Store, actually it's a Premium Reseller, we don't have Apple stores in Argentina.
Anyway, I left my MPB13 (250GB stock HD) three days ago, today they called me because it was ready. When I get there, they told me that they didn't find anything wrong with it. I've asked to talk to the technician, he said that it's normal to have the beachball from time to time. I explained to him that it's more than that and ran the AJA System Test (
http://www.aja.com/products/software/) where you can see that the HD performance drops to almost 0 MB/s when reading a 512MB file. After showing that, he took the laptop again. While I was waiting for him to return, I've run the same test on another MPB13 that they were showing. It didn't have the 1.7 firmware and the performance was a steady ~70MB/s writing and reading. Bottom line, they kept my MPB, they will look more into it and probably change the HD and memory (that's what he said).
So, it's not over but at least they recognized that something is not right. I've also mentioned this thread were they can find more information.
I recommend this AJA System Test, I've run it in several Macs and the performance is always constant not like my MBP13 with EFI 1.7.
They'll call me on Monday... I let you how it goes.