Most of us learned, probably in school, that a haiku is a Japanese poetry form in three lines 5, 7, and 5 syllables. This, it turns out, is a simplification and not necessarily true. It's a good firm guideline, a rule you can hang your hat on, but it overpowers the inherent subtleties of the form.
Recently, Peter Pereira pointed to a post about Haiku Blogging and offered that as a way to create short, compressed blog posts. Michael Dylan Welch then weighed in (yes, I'm kind of giving you the play-by-play) with some comments.
Michael knows a lot about haiku, and in his comments he explains that other factors are usually considered more important than a syllabic structure (Japanese doesn't parse by syllables anyway, but by sounds, or mora). These factors, he continues, are "kigo (season word), kireji (cutting word that usually divides the poem into two juxtaposed parts), and objective imagery (no concepts, judgments, conclusions, or analysis)." He adds that you can find a different haiku every day at tinywords.com.
For more information about haiku, you can read Becoming a haiku poet or Michael's ten tips, or you can visit his collection of haiku and photographs, Open Window.