I've used Klipper - it copies whatever I select and holds it, along with four or five preious selections. At least, that was the functionality of the applet in KDE 3.5. I have no idea if KDE 4.1 has a version of it, nor how it works.

Klipper works with all programs and Desktop Environments, so I'm sure it would work with Tomboy.

(yet! I will drag my desktop to a high speed area! Someday.)