Google for IrDA, and look at MCP2140 and MCP2150 device from Microchip.
There is a recent thread in this list on the IrDA topic.

2007/7/3, John Waters <john_fm_waters@hotmail.com>:
>
> Hi All,
>
> I want to build a device that will talk using Infrared beam to the
> build-in
> Infrared Port of the iPAQ Pocket PC. However, I'm not sure what protocol
> the
> iPAQ Pocket PC is using, as well as practically how to implement it. Could
> someone give some directions.
>
> Thanks in advance!
>
> John
>
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-- 
Ariel Rocholl
Madrid, Spain
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