The original GSM network - now referred to as 2G - was a voice network. That original network allowed the carrying of data packets at 9.6 kbps. When data became more important during the late 1980s and early 1990s the standard was upgraded and, with some minor equipment modifications, GPRS was introduced. Furtheer modifications to the 2G standard were later introduced with EDGE which used enhanced modulation techniques to further increase the data rate. Both GPRS and EDGE use the original - albeit modified - 2G network and were relative simple changes for the service provider.
The 3G network, however, required a completely new infrastructure - new basestations, new antennas etc - and required a very considerable investment by the service provider. Therefore, many took the decision only to install 3G in urban environments where they perceived the usage would justify the very considerable capital expenditure necessary. Therefore, if your iPad is displaying the 'E' (EDGE) symbol it means that you're currently in an area where AT&T have not installed a 3G network and there is no way that you can move to 3G unless AT&T take the decision to install new equipment there.
Next time you take your iPad to a different, densely populated, urban area you might well see that 'E' symbol change to '3G' and you'll then experience a very considerable improvement in performance. If you live in an EDGE area, then I'm afraid - other than getting WiFi via cable broadband - there's very little you can do about it - other than write to AT&T and see if you can persuade them to install a 3G network.
Tim