Hmmm, that's odd... Eight minutes ago, I started experiencing my first visual migraine. Luckily, my brother had one a little while back, so I know what's going on. It started as a little blind spot, that was making it hard to see the letter on the screen that I was looking at. It has since grown and moved off to the left. It's now like if I were looking out a window where someone had frozen the letter "C" onto the glass, causing a rainbow diffraction effect, except that the ice is also shimmering. It should end within the half hour, by all reports, and I might get a slight headache after. Very strange, and it would be scaring the tar out of me right now (think: stroke) if I didn't know what it was.
Update: It passed as predicted, in about a half an hour. I got the slightest of headaches afterwards, which is also typical. I found this Flash animation that gives a very good depiction of what it was like (except that the distortion was more of a "C" shape for me).