This is a quick demo of REAL WATERCOLORS FOR Ai's capabilities. I was asked how to create a watercolor fill effect. I've used the solid vector version (top) and the 10-color vector version (bottom) for this effect. The rest is masking magic. You'll find everything you need in the demo!
Here is the watercolor freebie file as plain vectors/images as well as brushes (check available ones in the brushes panel). This should work for you CS5 guys.
I've worked a bit on the combination of a vector brush masking out a seamless texture.
These results are quite cool:
I was asked if it was possible to prevent ugly distortions on short strokes. The Idea was to bypass the the event by creating a vector brush combined with a seamless texture. Yet quite rough but it works quite fine…