John, The following example works ok with WXAgg (Hoorray!)

    > but the exponent 4 at the end of the expression does not
    > show up (with or without brackets).

Yes, there is a bug in the parser for over/under subscripts. I think
I have the fix for it, but haven't been able get it done yet.
    > Also, an anoying thing is the need for spaces around TeX
    > markup. The same expression does not get formatted by
    > mathtext if you remove the spaces. This is going to
    > confuse TeX users since LaTeX doesn't need spaces.

Yes, at the least a \/ symbol would be nice.

    > one last thing: after you close the WXAgg figure, the
    > execution does not return to the interpreter. This does
    > not happen on GTK.

OK, thanks for letting me know.