The toolbar includes some of the usual StumbleUpon buttons, Stumble, thumb-up and share. URLs can be posted directly to twitter / Facebook or scheduled to post at a later time. This can be up to two weeks later. Here's an example of a shortened URL, http://su.pr/1JKt8R.
By using su.pr to shorten URLs, the pages you tweet can potentially benefit from additional StumbleUpon traffic. su.pr includes a range of statistics for shortened URLs, including clicks, traffic from SU, referrers, etc.
StumbleUpon are planning to automate su.pr to post items from an RSS feed. Until then why not try combining twitterfeed with the Digg toolbar?