I just got N9 and like the style of the phone and OS. I used Straight Talk pre-paid $45 unlimited Plan and was succesfully able to manually set the Internet APN settings under
Settings -> Internet connection -> Edit networks where it showed AT&T Internet by default on detecting my Straight Talk SIM. I edit that to "Straight Talk Internet" with APN value to att.mvno. I cleared the username and password to blank values and now my mobile Internet works.

I have two questions. First, in the status bar I don't see 3G, instead I see 2.5. Second, my MMS doesn't work as I didn't find way to add second APN which has different settings for Straight Talk.

Any help appreciated on where to add second APN for MMS and also if I am connect to EDGE or 3G.

[Updated]
I figured it finally.

MMS settings can be editing from Settings -> Applications -> Messages -> "Multimedia message center" and choose "User defined" with following settings:

APN: att.mvno
Username:
Password:
MMSC: http://mmsc.cingular.com
MMS Proxy: proxy.mvno.tracfone.com
MMS port: 80