I just noticed something in my testing. I was searching for questions tagged "C#" and the questions come back tagged "C".
Code Poet's SOAPI javascript formats the route thusly:
http://api.stackoverflow.com/1.0/questions?tagged=c%23&sort=activity&order=desc
His works, whereas mine includes the "#" even when I HttpUtility.HtmlEncode it. What am I missing, and is there a quick and dirty converter?