D3 Portable Installation on a USB Flash Drive
Diablo III plays wonderfully installed on a USB flash drive.
No lag or anything. (Well, no more than usual.) I already had D3 installed on my computer at home, and I just copied the entire Program Files\Diablo III folder onto a 16GB USB 2.0 drive.
This is useful for going to a friend's house, a school computer lab, your work computer... instead of having to download and install ~8GB each time, just bring your flash drive with you.
Anywhere there's a computer capable of running D3 you can just plug and play, and you'll find all your characters/settings/keybindings intact.
A note about space: You'll need more than 8GB. A "8GB" flash drive has an actual capacity of about 7.5GB, and the D3 client is already at ~7.75GB and likely to get larger from patches, etc., in the future. That's why I used a 16GB drive.
Hope this info helps someone else-- have fun!