To elaborate on my earlier post...

Both FTDI and Silicon Labs have USB-Serial chips that also include GPIO pins. Since you also have the modem control lines, these may give you enough control. The drawback is they are surface mount chips. However, Spark Fun Electronics has them on breakout boards. They have schematics on each product page.

Spark Fun also has the USB Bit Whacker which is a development board for PIC18F2553. It has numerous GPIO pins.

All the above are here...http://www.sparkfun.com/search/resul...&what=products

The product page for the USB Bit Whacker is at...http://www.sparkfun.com/products/762

All of these are seen as virtual serial ports on a PC. I've not tried the Bit Whacker but have used FTDI, Silicon Labs and Prolific chips and all work with Windows, Linux and OSX. Most are recognized by the latest versions of Windows and by the Linux kernel and you do not need to install drivers. OSX recognizes them but you must install the driver to use it.