Q: Is there a way to convert ArcGIS/ArcView symbols to SLD files?

A: Yes it is possible, thanks to a tool Albrecht Weiser wrote.

Here is his e-mail with a link to his tool:

You don't have to create SLD-files from shapefiles manually. As i posted a week ago, i've developed a tool, which analyzes an existing ArcMap project based on shapefile-, personal GDB-, or ArcSDE- data and puts out the styling-informations in an SLD-file which represents that project. The tool is called ArcMap2SLDConverter and you can find it here:http://www2.geoinform.fh-mainz.de/arcmap2sldconverter/ Unfortunately the site (and the tool) is still in german language because i was too busy to create a translation last time, but i think i a week or so i can present an english version too. You need to install a .NET 1.1 framework before you install ArcMap2SLDConverter. The tool must be started parallel to your running ArcMap 9.0-session. When specified the location to store the SLD and checked the option which datatype (shapefile or database) your WebMap-project is based on, you push the sld-button and everything runs automatically.