TI has just released an 8 bit parallel to serial converter with an SPI interface. Interesting thing is that they will interface directly to 24VDC inputs, have some debouncing and current limiting on the input. They have on on-board regulator for the SPI signals and are cascacadable. They also have drivers for external status LEDs for each of the inputs.

Check out the SN65HVS882 and SN65HVS880
http://focus.ti.com/docs/prod...vs882.html
http://focus.ti.com/docs/prod...vs880.html

Having been stung by TI's discontinuation of many products over the years I am a little hesitant, but the advantages may outweigh the long term possibilty of discontinuation. Of course if more of us use it then its success will ensure its viabilty.

-Aubrey