In an interesting follow up to the "beware if you install more memory to WP7!" advice from Microsoft, AT&T seems to have no problem with you doing so. If you recall, in the article, the responsibility was basically passed off to the carriers if they want to "support" such behavior. Microsoft had said that many of the microSD cards out on the market were unreliable and therefore may cause problems.
Over in the AT&T forums, user eshudnow emailed Jeff Bradley, Senior Vice President of Devices for AT&T about the issue and this is reportedly his response:
Thanks for your interest in our new Windows Phone 7 devices. We are very excited as well about this great new experience being delivered on some very cool devices on the nation’s fastest network.
The devices will support the addition of up to a 32GB class 2 (or higher) microSD card. You need to insert the card before you power up the device the first time so that the operating system can map it as available memory to maximize its utilization. This is outlined in the Quick Start Guide you receive in the box. I encourage you to read this before you launch the device the first time to have the best experience with a microSD card.
Thanks again for your interest. I hope you like it as much as I do. If you have a moment, please let me know your experience.
So there you go. The max (at this point) seems to be 32GB and AT&T has no issue with you doing this and in fact, they give you explicit instructions on how to go about doing it. Sounds good to us. Hopefully other carriers will take a more lenient approach. Though this still raises the question if there will be a "warranty void sticker" and if so, will AT&T still enforce that or not?