From http://groups.google.com/group/twitter-development-talk/web/api-documentation
, emphasis mine.
status. Required. The text of your status update. ***Be sure to URL
encode as necessary***. Must not be more than 160 characters and
should not be more than 140 characters to ensure optimal display.
From a quick google search for "url encoding"
http://www.blooberry.com/indexdot/html/topics/urlencoding.htm