If Gigabit must be in your short-term future, the good news is that NetApp clusters do support it and it sure is fast! The bad news (you knew it was coming <g>) is that we don't support multiple MAC addresses per NIC with our Gigabit drivers today and consequently you'll need at least two Gigabit interfaces (one active, one passive) per filer in a cluster config. Again, no word on when that will change.
It won't ever change, because the drivers can't do what the hardware doesn't support. We've looked at support for the AceNIC 2, which I believe can handle multiple MAC addresses in addition to having much higher performance, but the engineer working on that has been focused on higher-priority projects.
-- Karl