I'm sure it is mentioned in some of the threads linked to from here, but since no-one has mentioned it I believe in PS/2 mode the IPAC can handle more simultaneous keypresses (than USB).
I cover this. Before MinWah's get flamed, here's what I say (basically, it's true, but not a very big issue):
Depending on board revision, there is a limit of 14, 22, or 16 inputs (plus modifiers such as Shift, Alt, and Ctrl) that the I-PAC supports in USB mode, unlimited in PS/2. While this isn't a very practical limitation (4 players each simultaneously pressing diagonals and pressing Buttons 1 and 2 simultaneously exceeds it, but not much else), it is a limitation which is not present in PS/2 mode.