In the past month I've noticed that I lose 3G data service in lots of places, and more importantly in places where I used to have plenty of coverage. I decided to google "is sprint coverage getting worse" and learned that I am not alone. In the past couple of months there have been lots of similar discussion threads, e.g.
The really significant one seems to be this detailed post on the Sprint Community forum,
My initial suspicion had been that Sprint was preparing to announce their "new 4G service" here in San Diego, and that they were re-purposing existing 3G towers for the effort. Now I'm not so sure. Even in areas with existing 4G service, the coverage map has dramatically shrunk recently. The discussion on Sprint Community is too long for me to read right now but the gist seems to be that Sprint never really had full 3G coverage -- instead they were paying for 'seamless' roaming on other networks (cough*Verizon*cough) and now they're not.
I was already thinking of switching to T-Mobile for their better selection of Android phones and existing '4G' coverage. This is now yet another reason to do so.