IEEE 1394
...with great difficulty, i'm afraid. The Firewire interface includes a good deal of encryption and copy protection measures. In addition to that, it's a rather complex interface. Firewire is, in fact, a network interface - there are packets with addresses and priorities, and different possible logical interfaces which can be created between devices connected to the 1394 network: isochronous / asynchronous.
To use firewire, you'll need a PHY layer device such as the TI TSB41LV03, and an LLC (link layer controller). The LLC will interface with your microprocessor, probably using a DMA style interface. Then, you'll need to write some application code to handle the interface. The list of things to deal with just goes on...
So, all in all, I'd say that firewire is a little bit complicated for DIYers at this point. Hopefully at some point a company will release a simple to use firewire system-on-chip like the FT8U245 series USB chips (perhaps there already is one?).