At the moment, I use an E61i for a backup phone, and as a data transfer device.
Right now it has no SIM card, and uses Wifi-only for data. I use it to transfer data between my Oracle Calendar (SyncML only) and Google account (so my G1 can access that data, since the G1 doesn't currently have a SyncML client for calendar data). I pay $0/mo for this.
In the past, I had a pay-per-day SIM card in it (to use as a backup phone), which is a data-less service. So it was also wifi-only for data at that time. I paid $0/mo for GSM data when I was doing that.
Either way, no one forced me to buy a GSM data plan
Well, it is 99 dollars. Your bridge in Brooklyn analogy only works if you didn't have to pay for GSM data in the upcoming N900 - which you will. Either way, want to use anything other than wifi, you're paying for it.