Ok you have some options and you don't.

If you want to have your own connection (instead of relying on wifi) then you are going to be tied to a cellular company most/all of which require a contract (within the US anyway). Actually the trick is that most of them will not sell you a card unless you sign up for a 2 year contract but if you search craigslist or ebay you can pick one up and then bring it to your carrier for activation.

<EDIT>
I forgot to mention that if you are in the US (mentioning location helps; I am just assuming since you said AT&T) there is a 5GB/month cap for all US cell carriers. Thank the FCC and the cellular lobbyists for that one.
</EDIT>

If you are the malicious type go for a compatible ralink or atheros usb wifi dongle. See this thread. Note you may need to a battery-powered usb hub or a y-cable + AA/lithium powerpack. You will also need a microUSB OTG cable from ebay (or alternative solution) for any usb device.

Most people will refer you to a MiFi/cradlepoint router OR a bluetooth network-sharing cellphone which is the cheaper (and best wireless solution; no messy cables attached to N810). A list is available http://wiki.maemo.org/Bluetooth_netw...ed_cell_phones

If you insist on a USB aircard/3G modem then see http://wiki.maemo.org/3G_Modem which I think refers to cards that support the driver referred to here http://www.shapeshifter.se/code/hso/

Oh and on the phone/talk topic. Your best bet is Google Voice (request an invite now! it takes a few weeks to receive) piped through Gizmo. That way if you are connected to wifi (or one of the above 3g solutions) you can call out and receive calls from any cellphone/landline on your device. And if you aren't connected people can leave a voicemail. SMS is also supported. See dialcentral.

Good luck, and BTW if the successor the n810 ever ships it'll have 3g data support built-in. Of course you are better off snagging a N810 now for around $200 plus some accessories.