What can you do with IEEE 1394 port on LCD?  

I'm not sure if there is a PC that supports video out over an IEEE 1394 connection. One BIG feature that would be nice to have is something like a 1080p TV or HD TiVo that supports program out for archival purposes via IEEE 1394. But given this era of DRM and content "piracy" I doubt we will ever see this capability.

Firmware upgrades.

On Windows, it can be used to stream videos on digital channels from the TV to the computer using VLC. I haven't been successful get the stream out from the PC to TV though.

With a Mac, though, you can use the computer to emulate a dvhs device and I can both record to and play from my Mac using the iLink controls on my TV.

Tell that to Panasonic... its not going to happen. Where is my HD stretch firmware update?

My HD cable box has a IEEE 1394 port, can I use this to connect to a 37d90u tv?

It's a hit or miss proposition as to what the manufacturers intended. The first IEEE1394 connections on Sharp sets were for D-VHS and only D-VHS. Now camcorders have it as well as some other devices. You just have to experiment with it and read the manual.