The only time that you must have a data plan on your account when having a G1 or any Android device through T-Mobile is to get it at a discounted price. Plenty of people use T-Mobile Android devices without a data plan everyday. If you are willing to pay full price for an Android device, or have found someone to give you or sell you one for cheap then you can actually use this device on your service without a data plan. Most Androids have WiFi which can allow you to use your home network as data to reach the internet without a data plan through T-Mobile.
As for the MMS messaging, your MMS messaging may not be working right now, this is because you are running without a data plan at the moment. If you call T-Mobile and ask them this, they ARE going to tell you that you need the data plan to send and receive MMS messages, but this isn't true. We are told to say this because we have to upsell our data plans, especially if someone is using a PDA/smart phone without one.
Keep trying to send MMS messages to and from your friends, try sending some to yourself. If they don't come through, they will soon enough. It's simply because right now T-Mobile is trying to locate your device on the network and they can't do so very easily because you are using it without a data plan. Once you are located on the network, everything should come in fine, except for data unless you are connected to a wireless network.