Problems with security trimming can be easily tracked down by using a built-in control along with ours (e.g. asp:Menu or asp:TreeView). If they behave in the same way then the problem is somewhere in your implementation.
On aside note, security trimming is a feature of the site map provider - it should not be related with the control which is databound.
You can find more information in Google or read the following MSDN article. Also Scott Mitchell wrote a nice tutorial with source code.
Our navigation controls mimic the default ASP.NET menu in terms of sitemap support.
the Telerik team