I don't have much to add here, although I seem to recall from childhood lessons in Norwegian history that fish was definitely considered a "poor-man's" food. Also, reading about Viking explorations I got the impression they wanted to make a name for themselves. Thus, given our previous discussion about risk-taking and poverty they were most likely not poor but rather the Viking-era equivalents of spoilt rich kids with nothing better to do than to compete amongst themselves.
So my speculation is that it wasn't so much a "taboo" against eating fish (certainly not "taboo" in a religious sense) as much as a case of stupid pride: "We won't end up like those lowlife peasants back home who have to eat fish in order to survive".
Just my two øre.
PS: I noticed several "puzzling" claims when I read Jared Diamond's "Guns, Germs and Steel" a while back. I suspect he is the kind of author that is best read at a high level, without paying too close attention to every detail. So don't discount his broader arguments even if it does turn out he's a little off on those fishies...
UPDATE: I missed this comment to Matt's post by Lemuel Pitkin, he is basically saying the same thing: "the feudal structure of Norse society meant that political power was based on control over access to land".
UPDATE II: Commenter Roxy/Ragu at Anthropik has settled this issue once and for all:
Well, it seems that there is quite a bit of disagreement on this particular case. It's not that the Greenland Norse didn't consider fish as food - many Norse back home ate fish all the time - but instead ran out of supplies to make the boats they were used to fishing in, from a combination of outstripping their resources in Greenland, and the discontinuance of ships from mainland Europe (the Plague might have had something to do with this as well). Lacking supplies, the Norse were restricted to fish they could catch from the shore, a method they probably weren't very skilled at. The Inuit had no problems making kayaks or fishing, but The Vikings' view of the Inuit as being inferior "wretches" probably kept them from trying to learn anything from the natives. So they died out, perhaps from ethnocentrism more than a cultural taboo against fish, although if the Greenlanders were in fact upper-class and had an existing aversion to fish as a "poor man's food", that certainly wouldn't have helped.