Mine does this as well. I imagine it is because of the fact it is a global device. I also leave mine to EVDO as I don't see me ever using a SIM. It would be nice to not have it come up though since it has to be rebooted pretty much daily.
I had the same problem, then I realized that when the phone was first set up, I had selected not to change automatically, and then gone in manually and set it to 1xEV.
The next time it rebooted, I selected the automatically option and then went into options and changed it back, and it never asked me again. So I would recommend trying this. Hope it helps.