Here's how to get a Blackberry 8700c to work on an AT&T/Cingular account that has a data plan but not a "Blackberry Data Plan."

1. On your PC, upgrade/downgrade your Blackberry Desktop to 4.2.2.14

2. On your PC, with your Blackberry connected via USB, upgrade/downgrade your Blackberry Handheld to v4.2.1.96
3. Go to Settings/Options/Advanced Options/TCP
1. APN = wap.cingular
2. Username for APN: = wap@cingulargprs.com
3. Password = CINGULAR1

4. You might have to reboot your Blackberry at this point. (I did, but I don't know that it is necessary.)
5. Now you've got data access, but none of the Blackberry applications will use it because it isn't a Blackberry Data Plan.
6. On your PC, go to Opera.com and download and install Opera Mini v3, and install it using Blackberry Desktop.
7. Run Opera, and confirm that you can get to the Internet.
8. Now you probably want to add GMail.  GMail assumes you can do an over-the-air install, which you can't, because Opera Mini v3 won't do downloads.  (It calls upon the Blackberry browser to do them, and the Blackberry browser won't work without a Blackberry Data Plan.)
1. Borrow a SIM card from a Blackberry that does have a Blackberry Data Plan. 

1. Move it to your Blackberry that doesn't have GMail. 

2. On your mail-free Blackberry, browse to http://gmail.com/app and install GMail. 

3. Then put the SIMs back into their 'proper' phones. 

4. Now you've got GMail.
5. Note: In version 1.5.1, GMail added support for notify-on-new-email. In GMail, click once, select More, then Settings, then scroll down to "Notify me when there is a new mail..."

2. As an alternate, if you can't borrow a SIM, see http://www.blackberryforums.com/aftermarket-software/55917-new-gmail-app-bb-application-loader.html  , but it is more work.

9. I don't think you can turn off the "Data connection refused" message without turning off data (via Settings/Mobile Network)
10. UPDATE: Opera 4 mini works too. I added Opera 4 mini by doing:
1. Download BlackBerry_JDE_Components_4.2.1.exe
2. Download Opera Mini 4 (opera-mini-4.0.10031-advanced-int.jad and opera-mini-4.0.10031-advanced-int.jar)
3. Run BlackBerry_JDE_Components_4.2.1.exe. If you don't have a JDK, de-select that prerequisite failure.
4. rapc import="c:\Program Files\Research in Motion\BlackBerry JDE Component Package 4.2.1\lib\net_rim_api.jar" codename=Opera4 -midlet
5. jad=opera-mini-4.0.10031-advanced-int.jad opera-mini-4.0.10031-advanced-int.jar
6. javaloader -usb load Opera4.cod