Also look at recording and music-related hardware items for iPad.
(The Dock is used by different manufacturers for digital audio in/out, midi etc.)

Apogee Jam
IK Multimedia iRig midi
Alesis IO dock
iConnectMidi
Cambridge Audio iD100 (TOSLink out, S/PDIF etc.)
Tascam iM2 (stereo condenser mic)

IK Multimedia iKlip
K&M iPad 2 stand holder
Apple digital AV adapter (HDMI out)

(very few equivalents on Android to date - Dec 2011, however there are some generic tablet holders for mic stands etc. coming out now.)