Sven

@CUP:
It's actually a 3D picture, it's just that by default it has no shading and is looking from the XY direction. Try this:
pts = rand(10,3) * 10 ;
growbubbles(pts)
view(3), axis image, camlight

@JD:
The issue is that your input is "gridded data" not "triangulated data" because each of x, y, z is a 20-by-20 matrix, rather than a set of vertices and triangles.
This can be handled by stlwrite (see the help file for the "triangulation" parameter), but you'll need to understand that the default "triangulation" of gridded data is a delaunayTri of the X-Y 2D data. In this sense you have repeated X-Y pairs (one at the top of the sphere, one at the bottom). The warning you're getting comes from "delaunay", not from "stlwrite".
You can overcome this by specifying how you want your gridded data triangulated:

stlwrite('test1.stl',x,y,z,'mode','ascii','triangulation','f')

Note that you'll need "mesh2tri" from http://www.mathworks.com/matlabcentral/fileexchange/28327 for this to work. I will ask Kevin for permission to include it in stlwrite (it's a small but very handy function) for a next release.

My FV has only 63 points and 122 triangles. But it took me 6.9 seconds to analyze 36144 query points and produced the wrong output.

@CUP:
It's actually a 3D picture, it's just that by default it has no shading and is looking from the XY direction. Try this:
pts = rand(10,3) * 10 ;
growbubbles(pts)
view(3), axis image, camlight