We had a client who is using the WP125 Ads plugin. When they upgraded to WordPress 3.0, it was having an error. I fixed the error and forwarded it to our client. Other folks reported the same issue in this thread in the WP support forums. The author of the plugin posted and indicated he was going to be tied up for a bit. I have forward the update to the plugin author. At some point he will be releasing an updated version.
In the mean time, you can download my updated version: WP125 Ads Plugin (422). If you have issues with it, please post those issues in the support thread linked above. I won’t be attempting to do other bug fixes or CSS changes.