If you have access to wifi, you can put the phone in airplane mode and enable wifi.
Heh. I always do things the hard way, and end up standing in the slowest grocery store aisle, too.
Your way is faster, easier to remember, and produces the same result - good call.
If I turn the "radio" off will this still let me stream the music with 3g connection or would I have to use wifi?
I think you will have to use WiFi. ( Alternatively, see if VZW has a (paid) service for call forwarding. Or what the heck, load music on to your SD card, turn off the radio, and use your phone as an MP3 player. )
Both the above suggested methods ("Radio off" / "Airplaine Mode" + WiFi ) turn the cellular radio off - which kills off both 3G and voice service.
What you are asking for might be technically feasible - for instance, the Verizon "MiFi" hotspot/cellular modem can not receive a voice call, even though it actually has a telephone number assigned to it. (You can send text messages to it, though... which are not 3G).
But, for the Eris, I haven't seen any sort of widgets or other software that allow you to adopt that usage model.
eu1