What are workable/straightforward methods for using external game controllers (bluetooth, USB) with the Android platform?
The best option (albeit a rather pricey one) appears to be the iControlPad.  It was originally designed for use with an iPhone but now uses bluetooth to communicate with all bluetooth-enabled devices.  It sounds like developers plan on supporting it, but I'm not sure how well it will work with current games.

EDIT:  I recently became aware of another bluetooth gamepad that caters specifically to smartphones.  It's called the Bluetooth Game Gripper.  There is an Andoinica article about it here, and the kickstarter project is here.  However, it doesn't appear that the maker has garnered enough VC to get the project off the ground, so I'm not sure of the current status.