Dewald Pretorius came across and shared on the Twitter dev-talk list the recently created Twitter engineering blog. Interesting engineering stuff to be read there, and not necessarily Twitter-specific by any means.
One of the entries mentions Twitter Text libraries for Ruby and Java. These libraries allow external consumers to, in theory, parse tweet text for URLs, screenname mentions and hashtags the same way Twitter does.
I grabbed Matt Sanford's twitter-text-java and created a quick C# port, NTwitterText. It looks like one or two of the URL regex patterns need some tweaking, but otherwise all functionality ported over pretty easily and cleanly.