Well, i-link is almost useless right now, but it is firewire and could be used to transport DVD-A and SACD at some point.
Huh? i.Link and Firewire and IEEE-1394 are the exact same thing. Right now, you can buy players and receivers (and a pre/pro) that offer i.Link connectivity from Yamaha, Sony, Onlyo, Denon, and Pioneer. Right now. And it supports the native transmission of DD, DD EX, DTS, DTS-ES, CD PCM, and all variants of DVD-A PCM (except for Sony), and ... DSD for SACD. Right now.
HDMI might someday offer the same digital audio transmission capability, but current components with HDMI will probably not be upgradeable.
And, i.Link provides for a jitterless digital transmission of audio signals. HDMI cannot and will never be able to do that. (Basically, both components clocks can be sync'ed with i.Link. The HDMI people had no such forethought.)