it can be. with verizon they push a vvm app to their phones that i believe is 1.99 a month. but you can also download various free and some one time payment vvm apps from appworld. i personally prefer using the vvm provided by verizon even though its 1.99 a month
Call AT&T @ 1-800-331-0500 or 611 from your wireless device and have them add the Blackberry Visual Voicemail to your account, there is no Monthly Service charges for the feature it just needs to "provisioned" to the account
The reason why 2G BlackBerry devices are excluded is because it would take forever and two days to download long voicemail messages, and incoming calls would interrupt the data connections. It's not compatibility, but practicality.
There is a code that has to go on your account to make it work but make sure you check your voicemails before you do it because they will all be lost and you will have to re-setup your VM