Sprint has network problems. Major problems. And they've gotten a lot worse lately. Really, really bad. Not all areas are affected - and in fact some have improved already, but more and more areas are getting so bad that Sprint's 3G data is completely unusable there, especially since the introduction of the iPhone. Troubleshooting and update my phone's "profile" and PRL didn't help, as evident from the screenshot #2 you see below.
Earlier this week I contacted Sprint's customer service, followed by an email to an executive and CEO Dan Hesse himself (or whoever fields his emails). The former told me there was a tower outage in my area, and a fix was incoming the next day (as you've guessed nothing is fixed as of today, 5 days later).