expression(pi) produces not-equal symbol in X11 plot

On R 2.10.1 on OS X (10.6.2) using an X11() device,
plot(1:10, 1:10, main = expression(pi))
makes a not-equal symbol instead of a pi symbol.

Other Greek letters (including capital Pi) seem to work.  On a quartz device the pi symbol is displayed correctly.

I don't know if the issue is on the R side, but I post it here in case anyone knows where to go with it.

Perry