fkeel:
I have been playing around with the GPIO pins. For me those are the most exciting aspect of the pi. Guess I will eventually connect some nice ADCs vie I2C.
Yeap I agree with that. For some like me who likes hacking hardware the GPIO are the RPi's most attractive feature 
The APC IO looks a great board, but there's no info on what GPIO is got. Connector J3 is a mystery, perhaps that's got GPIO or I2C.
fkeel:
question to you guys: If you could mod the pi (or if you will mod the pi) what will/would your first hardware ad-on be?
Already done my first, second and third addon board for the Pi: Prototyping Board. BBC like I/O User Port. MIDI Interface.
Just waiting for my PCB's to arrive and then my old soldering iron won't know what's hit it