Because there aren't any up-to-date guides on how to do this, I decided to make my own.
All you need is an unactivated (never used) AT&T SIM card.
To start, go to http://www.att.com/ipad.
Scroll down to where it says "Need More Data?" Click on that link (or click here, but this direct link might not always work).
Click "Get Your Pass."
You will come to a "Device Information" screen. Here is where you will enter a fake iPad IMEI and your unactivated AT&T SIM number. The IMEI should be in the form of 01222300******* or 01222400*******, where ******* is a random 7-digit number. The SIM number should be right on your SIM card.
As an alternative to guessing IMEI numbers, you can also get the IMEI from an iPad Wi-Fi + 3G in your local Apple or AT&T store. Just go to Settings->General->About to see it. Taking a picture with your phone would probably look less suspicious than writing the number down, but either way YMMV.
Click "Next." If it doesn't work, try again with another fake iPad IMEI. I got my first number to work on my first try (what are the chances), but the second valid IMEI I hit took over 20 more tries to get, so keep trying till you get it!
Once you get past this screen, you're home free! All you need to do is enter your name, address, email, password for your new AT&T account, Credit Card info, agree to the terms, and you're set. Also, from now on, you'll be able to manage your account from the email and password you set up earlier.
Have fun with your new data plan!