Spot posted an ambiguous entry, which happened to coincide with some kind of Xft2 bug in Rawhide, which makes
subpixel rendering to show colorful outlines of letters:
Peter Jones said in comments that the rainbows are not seen in screenshots, but of course they are. Look at the 'i' in Nigorimasen.
UPDATE: Just look at it with xmag already:
See that blue strip?
UPDATE: The subpixel rendering is actually off, the GNOME preferences are set for "best shapes". The problem is that they use colorful dots instead of grayscale ones. There must be some kind of reason why they decided to do it, and perhaps it is actually better on some CRTs. The fact is, everything looked fine until the last "yum update", and now it's hideous.
UPDATE: My word, they did it on purpose!