Scott,
You can do exactly what you want with this bit of code from Twitter. The example below opens up a Twitter share dialog and populates the tweet with the title of a Marketo blog, a link to the blog, and a Via @Marketo at the end.
<a href="http://twitter.com/share?url=http://blog.marketo.com/blog/2013/01/if-your-writing-sucks-so-will-your-content.html&text=If Your Writing Sucks, So Will Your Content&via=Marketo&related=yarrcat" target="_blank">Tweet This button</a>
Kelly