The performance testing they did just completely confounds me, however-- in situations where lots of memory was needed (the "RAM Hog" setup), 4GB clearly beats everything else. In fact, in most of the regular testing-- the Photoshop test and the "Stress Test" the memory runs exactly how you'd expect: more memory means quicker processing. But in the last two tests, the Halo test and the Xbench test, the mini actually ran faster at just 1GB (which is what it ships with) than the 3GB, and almost as fast as the 4GB. The reasons for that are probably technical (too technical for me, although I'm sure our talented commenters have some answers), but it seems that in some situations, extra RAM just doesn't help.
Still, in situations that ask for a lot of memory, having more will definitely give you a nice bonus in speed, and it looks like the mini can now hold more than ever. When I buy mine (in October when Leopard drops, assuming Steve lets it live that long), I'll probably just double it to a reasonable 2GB. But if you want that extra power, it's good to know it's there.