Before I start, I am getting HSPA+ speeds of around 12MBps, I have full working data, text, calls, and MMS. At the moment I haven't been able to get the tethering to work, wifi or otherwise.. says number not found. I assume that means its checking to see if its available for a plan that allows it via AT&T.
APN settings I used are as follows.
Name: ATT HSPA+
Proxy: <Leave in Blank>
Port: <Leave in Blank>
Username: <Leave in Blank>
Password: <Leave in Blank>
Server: <Leave in Blank>
MMS proxy: proxy.mobile.att.net
MMS port: 80
Authentication type: <Leave in Blank>
APN type: default,supl,mms,hipri
APN Protocol: Leave it to the Default one
Bearer: leave it to the Default one
For those of you unfamiliar with how to change the APN settings: apn settings for Android - For Android, you should go to Settings > Wireless Connections > More > Mobile Networks > Access Point Names.
Its fairly straight forward. Make sure that once your finished adding the apn info you hit your menu key and save, then back out and select the apn you want to use.
APN setting source: http://apn-settings.com/us/att-apn-settings/
There are a couple video's on that site as well if you need a walk through.
I take no credit for the video's or the website, I am only relaying some useful information that I had trouble finding.
Hope this helps.