I don't know if this will help you but my wife started receiving these messages on her cellphone and we could not figure out why. Blocking texts from that number did not help. I had been using my wife's old phone as a wifi device while my phone is being repaired, without actually activating it, and since the problem had been occurring since I started doing this, I was thinking there might be a connection but could not figure out what it was. Then this evening I was in the room at the same time as my wife got a voicemail notification on her phone, and at the same time I got a notification on her old phone too! This seems very odd because her old phone should not be associated with her number any more, but it seems to be. Then I figured out what was causing the messages. Every time I switch on her old phone, she gets the text message from 8737 on her new phone. I was able to reproduce this three times in a row this evening.
In the cases here, it seems slightly different because you're actually receiving the text messages every time you reboot, but maybe what I have shared with you here will help you to figure out what's going on. It is a text message that is automatically generated by Sprint every time the phone is switched on. I would guess that the phone is supposed to catch that message and not display it but something on your phones (Handcent?) is preventing the message from being suppressed. In my case it's not being supressed because a different phone is causing the message. It may actually be related to Google Voice since I have Google Voice on the old phone (I googled "8737 Google Voice" and the results suggested there might be a link also).
This is actually quite troubling because it means that if you send a phone back to Sprint and someone does the same as me - i.e. use the phone's wifi features without activitating it - then you are going to get these weird messages on your new phone every time someone switches on your old phone!