A: 

have a look at these two pages:

http://stackoverflow.com/questions/475278/asp-net-mvc-and-css-having-menu-tab-stay-highlighted-on-selection

http://stackoverflow.com/questions/287679/an-easy-way-to-set-the-active-tab-using-controllers-and-a-usercontrol-in-asp-net

Mauro
Hmmm.. both solutions presented there are huge and tedious in comparison to the seemingly simple task :(
Alex
Both of those seem relatively straight forward. Do you not like the hard-coding?
Martin
+1  A: 

Would a solution such as using jQuery to highlight the active tab work for you?

You could select the <a> that contains the href attribute that equals the current page, and add a class to the parent <li>.

This is the simplest solution I can think of.

Chris Missal
You could probably do the same thing on the server side ... check the request, runat=server on the ul, and add a class to the correct li
Martin