<p>I would like to implement a sliding notification bar as the one in Firefox or IE for my java application. But I don't want to reinvent the wheel and I'm sure someone out there has already done it and is willing to share.
Do you know any open-source implementation of this in java/swing?</p>

There's an example in
Swing Hacks
called "Slide Notes Out from the Taskbar" that seems pretty close to what you want (there's a
preview
on Google's Book Search at the moment).
The code is freely available from the
official O'Reilly Swing Hacks page
, under "examples."  Not sure what the license is, but it's worth a look (Chapter 6, hack 46).  At any rate, Swing Hacks is a worthy purchase at ~$20.