Before bluetooth was around, the ability of connecting cables was a big issue, especially for users who wanted to connect barcode scanners. One of the biggest problems associated with connecting devices is that there is not enough power flowing from the cradle connector to supply power to the attached peripherals. When I used my old Palm III, there was a modem I could snap onto it. However, the modem had it's own battery installed.
If you really want to use a mouse, keyboard, barcode scanner, etc, you will probably need to find one with wireless capabilities such as bluetooth. I don't know if bluetooth mice exists or not, but keyboards and barcode scanners are in the market.