I spend almost half an hour to find how to do encoding\escaping special characters in SL. Started with SecurityElement to HttpUtility etc but none were available in SL. Well as it is with google, if you ask correctly you would be rewarded. Turns out that there is a class in System.Windows.Browser assembly for SL that contains HttpUtility class. Checkout here

If i had just type HttpUtility in the IDE i could have got it :(

Anyhow, Happy Coding.