Bought my phone at Best Buy. And by "bought" I mean "I got the Fascinate last year, a month after release, for free, on Free Phone Friday"
Recently got upgraded to the Droid Charge BY VERIZON, not Best Buy, as part of the exchange/upgrade deal that they were working b/c of all of the Fascinate problems. So I had no problems using Verizon, even though I bought it somewhere else.
Also, one thing I found out is that some retailers claim to have lower/no deductible, however the reason they can claim that is because they won't even cover the things that Verizon' insurance will cover.
Example: Best Buy says they do not have a deductible. Verizon says there will be a deductible if you drop your phone in water. Best Buy will not warrant a phone that is fully submerged. Therefor I'd rather get my phone replaced with Verizon if I paid a 100 dollar deductible, rather than having a broken phone that won't be fixed if I was using Best Buy's protection.