I bought my phone at a Sprint store on 1/2, which is over the 30 days. I called *2 and was shot down by the CSR. I was very polite (and even a bit chatty) and asked if there might be a chance her supervisor or manager could offer me credit. She said she'd be happy to check, put me on hold, came back with a few questions, put me hold again, then credited my account with $50. Bata-bing. Bata-boom.
Oh, and I had previously negotiated $100 credit over 2 months as a "price match" (well, almost) with Verizon.
I'm on Sprint SERO, BTW.