The URL of a subsite may appear in uppercase even though you used lowercase when you created the subsite in SharePoint Server 2007
You may find this problem being arised by your customers. You can find the description in the KB: http://support.microsoft.com/kb/953457/en-us
As you can read in the BCL recommendations at http://blogs.msdn.com/bclteam/archive/2005/06/01/424012.aspx
DO: Use ToUpperInvariant rather than ToLowerInvariant when normalizing strings for comparison.
The problem here is that the key is used as the value too, so navigation control shows uppercase title sites.