Another update:

As suggested earlier, I changed both of the network connections to "WORK" and this, indeed, cuts off MMS access. I needed to send a picture message on friday, and the message would immediately fail after being generated. After changing the connection back to Internet, it sent on the first try.

Despite the obvious implications with MMS, I think that changing the networks to WORK is a good option to prevent the phone from accessing the EVDO (BroadbandAccess) network and charging up the bill.

I try to change the networks to WORK before using my phone in a wifi hotspot to ensure I'm using wifi only... but I can't keep it this way all the time, since I never know when I'm going to receive an MMS message.

I'm waiting for some sort of magical app that can discriminate between MMS (free) and EVDO/BBA (paid) access, and allow the connection accordingly. It doesn't seem that difficult to program... but I know I couldn't do it. Any takers?