Well, will you look at that.
How to use the Verizon Droid DNA on AT&T (or any other GSM network) | Android Central
Here are the APN settings I used for AT&T.
Code:
Name: ATT PTA
APN: pta
MMSC: [URL="http://mmsc.mobile.att.net/"]http://mmsc.mobile.att.net[/URL]
MMS Proxy: proxy.mobile.att.net
MMS port: 80
MCC: 310
MNC: 410
Auth: none
APN type: default,mms,supl,hipri