I found a good explanation of encoding twitter status URLs. In short, there are a couple things you’ll need to do:

• Make sure your URL is pointing to http://twitter.com and not http://www.twitter.com
• Make sure you’re using + for spaces and not %20

If you try to URL encode your status message and send that tohttp://www.twitter.com, Twitter it will redirect to the non-www URL and re-encode your already encoded URL, thus encoding all % symbols and ruining your lovely message..

http://www.kilometer0.com/blog/2010/01/21/twitter-status-urls-and-ampersands/