The FAQ for MetaPost drawing is why ghostview cannot preview the picture but you can still included in LaTeX. Actually, this is due to the fact that, MP output is not a completely valid postscript, but just a partial file. Whenever you included some text, it cannot understand where to find the font as the font-related header is missing. No way to fix, this is a feature!
However, I know it is annoying and it make some begineers confused. But I found
two neat tools. First one is the
mptopdf command, which is bundled in my
Debian. It can convert the MP file directly into PDF pictures. Suitable for
pdflatex users. Another is the
mptoeps Perl script, available at
http://www.alcinoe.net/perl/mptoeps, which do the same thing but for a completely
valid eps file. This probably help to relief some people’s pain.