Thanks to comments on my blog from Ray (who I think is the same person as the R4Y who has posted here) I've learned about an interesting bug. It occurs in both WebKit and Firefox, and only on machines with certain ATI graphics cards, which makes it sound like a driver bug rather than a problem with any specific WebGL implementation.
The problem is that if you use vertexAttribPointer to specify your vertex positions, and then vertexAttrib4fv to specify their colours (as you naturally might if you were drawing an object that is all the same colour), you get nothing drawn. This showed up in my second tutorial (http://learningwebgl.com/blog/?p=134) and the succeeding ones, as in them I was using that technique to draw a blue square.
I suspect you'd get something similar if you used vertexAttrib* for any vertex attribute, though I haven't done any exhaustive tests.
Anyway, I'm just posting this here as a warning. I've changed all of my own demos so that I specify all attributes per-vertex, even if they don't actually change between the vertices.