[vhawk01]

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So macroevolution is just microevolution times X, where X is defined as "whatever number is big enough

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Not exactly. Macroevolution deals with evolution beyond individual species. We can take microevolution times X for X as big as we want, and it isn't sufficient to be in the domain of macroevolution (although it is necessary). The other definining criteria is that speciation has to occur. This is because macroevolution deals with topics like diversity of species (which, of course, speciation is necessary for). I think there may be other ways that people use the term macroevolution, but those seem to be confusing and incorrect.

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Macro is more than micro, sure, ok, but how much more, and what is the fundamental difference between the two that leads to the idea that one is possible/has happened and the other hasn't?

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There is no fundamental difference between the two. I think the idea that one is impossible comes from a misunderstanding of evolution (albeit an extremely subtle misunderstanding of how speciation occurs). If we assume that microevolution occurs in a population, and that reproductive viability is dependent upon heritable characteristics (genes), then speciation is inevitable. Microevolution leads to macroevolution.

To easily see this, just split a population up into two groups. If there is no gene flow present between the two groups, then due to genetic drift they will diverge genetically over time (this process is random, or at least partly random). If we acknowledge that there are genes responsible for reproductive success, then these genes will diverge over time as well. Since there is no selective pressure for population 1 to be reproductively viable with population 2, over time they will no longer be reproductively viable with each other. This is simply due to random mutations in the genome, which we already acknowledged occurs (microevolution). These simple changes in allele frequency lead population 1 to differ from population 2, and the genes responsible for reproduction diverge via the same simple mechanism.

Of course once there are no remaining members that can interbreed, then over thousands and thousands of generations they continue to diverge (no gene flow). Give it long enough, and the two species look nothing alike. Now just repeat this process over and over again for millions of years, and you can see why we have so many species that look so vastly different.

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I think we agree on pretty much all of this, which again makes me wonder why you objected to the numbered points 2) and 3) above. Aren't you essentially agreeing that the only difference between microevolution and macroevolution is that macro involves speciation events? That was what I offered as the only non-arbitrary division between the two, and it seemed to be what hitch was trying to say, but then it seemed like you disagreed with that for some reason.