It's funny, I noticed this a month or so ago in Shreveport Louisiana. I live in an area where Shreveport is the closest 3G network. At home we have horrible signal fluctuations, but when it works Edge is mostly fine. Before Shreveport went 3G I would get 5 bars and Edge was pretty good (for Edge anyway).
After the 3G switch, I still get 5 bars of service, but the Edge symbol almost never comes on, instead I get the weird little 'dot in a circle' that tells you you are one GPRS, and with a 1st gen iPhone that means no data whatsoever. Calls are great though.
Occasionally the E will appear for a short time, and when it does it is like the Edge network that was there before 3G came. But it only lasts seconds, or sometimes maybe minutes, then goes away again.
At least with this setup I know out of the gate I'm not going to get service when the edge icon is completely missing.
The first time I noticed this I was with some people who had 3G iPhones. With the 3G disabled their phones were doing the exact same thing, so I know it isn't my phone being weird.
This is one of the few times I feel lucky to be nowhere near 3G service, as it would make my fully functional phone not work properly, and I'd be 'incentivized' to upgrade. Now I can keep my working phone, and slightly less expensive data plan for the time being.