Work long enough with Flash and dynamic textfields, and you'll probably run into this one—words that appear to have an extra and/or missing space preceding or following them, often when a link is introduced via the <A> tag. It doesn't happen with every font (I was using Helvetica Condensed—other condensed fonts seemed faulty as well), it doesn't happen with every link (certain letter combinations have a high degree of reproducability, others no problem).
I spent half a day narrowing it down. Since my text was originating from XML, it first masqueraded as some sort of entity or character encoding problem. I messed with TextFormat and StyleSheet (mutually exclusive by-the-way). A later wild goose test-case showed that flipping the autosize flag off made the issue disappear. Fine, but I need autosizing.
When Andreas Heim pointed out the solution, I remembered I'd run into this before: "Do you happen to be using 'Anti-alias for readability?'" Sure enough, flip it to "Anti-alias for animation" and all's well with the world.
I can only hope that the Flash 10 player addresses this bug. With all the work that's gone into its type rendering features in this revision, I'd certainly assume so. I'll post some test cases / screenshots in the next update.
Update: Screenshot of effected swf. Verified broken in Flash 10 beta player.