Dear Till/Christian/Hans,

The following is broken in ConTeXt MKIV (assuming the usage of patched
TikZ to allow running it under MKIV at all), but works fine in ConTeXt
MKII:

\usemodule[tikz]
\starttext

\starttikzpicture
   \node[circle,ball color=red!10] (a) at (0,0,0) {$p_x$};
\stoptikzpicture

\stoptext

Mojca