Many of the wireless routers are miniature Linux boxes and you could at least find a lot of information about how to add additional I/O. The Netgear WRT54G is quite popular, very hackable, and cheap (all of $70 at Wal*Mart!).
I don't know any particular, but some speculation here:
It's likely that there is a CPU hidden in there somewhere, but those adapters are usually complete 'systems on a chip' and unlikely to have very much spare I/O. It's also not uncommon that the chip vendors purposely don't document much of anything regarding how to use the CPU; they often provide a binary file and tell you how to dump it into the chip via USB, but that's it.
(I looked at an Atmel USB host IC once that was meant to be used with an ARM processor. While it was clear that the USB IC had a processor inside -- either an AVR or 8031 core -- Atmel just gave you the code that the ARM loaded into the thing and that was that. They actively refused to provide more details on its internal architecture/operation.)
I get enough excitement just figuring out how various manufacturers add in extra features such as icon control to LCDs that are otherwise 99% 'Hitachi HD44780 compatible.' :-)
---Joel Kolstad