<p>In a prior question, I asked about using xbuild to compile a solution file, only to be told it doesn't work for that.  NAnt seems a little out of date.  What other options are there?  I need something that supports dependencies across multiple projects.</p>

I'm not sure there's only one correct build system to use. :-)
I know that the Mono team uses make to build MonoDevelop and that MonoDevelop uses make under the covers to build projects created by MonoDevelop (
http://tirania.org/blog//texts/md-build.html
).
That said, there's also xBuild (
http://www.mono-project.com/Microsoft.Build
) which is an MSBuild compatible build engine created by the mono team.  That might actually be the best tool as it would allow you to build your projects under Microsoft's .NET platform in addition to Mono.  This link (
http://go-mono.com/forums/#nabble-td15680662
) talks about MD moving towards MSBuild also.