The following article says the second generation an later iPod Touches support the same bluetooth profiles so a keyboard should work. However, not all BT keyboards work.
http://support.apple.com/kb/HT3647
This article says you should consult with the keyboard manufacturer to see if it is compatible.
http://support.apple.com/kb/HT4112
The Users Guide provides the following instructions for Apple keyboards:
Pair an Apple Wireless Keyboard with iPod touch:
1 In Settings, choose General > Bluetooth and turn Bluetooth on.
2 Press the power button on the Apple Wireless Keyboard to turn it on.
3 On iPod touch, select the keyboard listed under Devices.
4 Type the passkey on the keyboard as instructed, then press Return.
Note: You can pair only one Apple Wireless Keyboard with iPod touch at a time. To pair
a different keyboard, you must first unpair the current one.