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

Not sure if you figured the 3G 2G part or not so.....

To get it off 2.5, go to
settings -> mobile network -> network mode and change from there.

You have dual, 2G only or 3G only, I personally have my phones set to 3G only as 2G is ancient tech, internet 3G vs 2G is like broadband vs dial up.
Battery standby does benefit from 2G mind you.

Try choosing between 2G 3G on iOS or Android, you cant.

I got my MMS now.