It seems that with the release of Leopard, the way iChat sends urls in private conversation and conflicts with the way Spark parses them. In iChat one can drag-drop a web page url and what the person PASTING sees in ichat is the description (title) of the web page underlined. What comes out at the other end is something like the following: text + square bracket + URL + square bracket for example:
Google + square bracket open + http://www.google.com/ + square bracket close
In Spark, nothing happens when clicking on that. This is with both 2.5.7 and 2.5.8 beta 1. I couldn’t figure out how to escape the brackets so I wrote them out in this example.