Adobe has announced a potential security risk in versions of Flash earlier than 10.0.45.2. This includes the versions in Lucid’s default repositories.
If you’re feeling paranoid or would just like to try the latest Flash 10.1 release candidate, you can download it from Adobe, and follow the install instructions from Web Upd8.
Update: The final 10.1 release from Adobe has hit the main Ubuntu repositories. A software update should do it.