I found this in the link that Chook-A-Holic posted, and this sounds like what I have, so I'll be curious to find out what the doc says:
People with ocular migraines can have a variety of visual symptoms. Typically you will see a small, enlarging blind spot (scotoma) in your central vision with bright, flickering lights (scintillations) or a shimmering zig-zag line (metamorphopsia) inside the blind spot. The blind spot usually enlarges and may move across your field of vision. This entire migraine phenomenon may end in only a few minutes, but usually lasts as long as about 20-30 minutes.
Generally, ocular migraines are considered harmless. Usually they are painless, cause no permanent visual or brain damage and do not require treatment.