How to export styles from ArcGis 9.3 (or 10) to a .map file. Is it possible?
There is a good discussion on sharing layer styles at http://www.spatiallyadjusted.com/2009/04/21/sharing-cartography/
SLD seems to be the only symbology standard that works with most GIS software. The downside of SLD versus a MapFile is the latter can easily be edited by hand, created by simple scripts, without all the XML structure that accompanies SLD.
If you wish to stick to MapFiles there is an ArcGIS script to export to a MapServer MapFile - http://arcscripts.esri.com/details.asp?dbid=12766
I'm not sure how well this deals with complex symbologies though.
SLD Export Options
You could try first exporting your ArcGIS symbology to Styled Layer Descriptors which are compatible with MapServer.
There is a free Arc2Sld tool at http://wald.intevation.org/frs/?group_id=32&release_id=213
I've read it also is not compatible with ArcGIS 10, but there is full source code so this could be updated. The code itself is in VB (and German). It was written by the University of applied Sciences in Mainz, Germany. Last bug fix in 2008.
There is also this functionality in arc2earth - http://help.arc2earth.com/kb/arc2earth-desktop/export-map-symbols-to-styled-layer-descriptor-sld It exports 1 layer at a time in the free edition, the paid versions can export a whole map file. The cheapest of these is 474 EUR.
It would be interesting to compare the outputs of both tools to see if there are differences.
Geocat Bridge version 1.2 is available on request at http://geocat.net. With this ArcGIS desktop extension you'll be able to export ArcGIS styles to Mapserver Mapfiles (used to be only available for Geoserver-SLD).