I encountered a curious problem today and I would like to share the solution for anybody else who is experiencing it.
There seems to be a bug in trunk/mcs/class/System.Web/System.Web.Compilation/BuildManager.cs on Mono whereby mod_mono disregards mod_rewrite's rewrite rules.
The output of my rewrite log indicated that the rewrite was working fine, but mod_mono disregarded this and processed the original URL.
The solution is, in all simplicity, to add the [PT] flag to your rule:
RewriteRule ^/(?!Default)([^\./]+)\.aspx /Default.aspx?action=$1 [PT,L]