I recently started to get some ads in the notification bar on my Nexus One. It turns out it was from an app I don't remember installing called Gtalk Updater and that it was using a framework called Airpush. I didn't like them one bit. In addition to just being annoying by pushing to the notification bar, they also caused my phone to vibrate.
Luckily someone has created an App called Airpush Detector which will find apps that are causing this update and take you to the uninstall screen for them.
If you are noticing ads in your notification bar on your Android phone, you may want to check this app out.