We recently fixed out 1.5 compilation errors. The only hiccup that you should encounter is that GeoServer assumes xalan (an apache xslt library), is included in Java. In 1.4 it was, but in 1.5 that appears to not be the case. To include it in the build targets edit the build.xml file and remove (or comment out) the line that says:

<exclude name="**/xalan.jar" >

This should be in the 'war' section of the file. Subsequent builds will then properly copy xalan to GeoServer.