1. right click on an unused portion of the task manager tool bar

2. select "task manager settings"

3. uncheck the box that says "show tooltips"

if you need to see a pic, try searching for taskmanagersettings.jpeg on docs.google.com

This is still an issue as of KDE 4.10.1. I can turn off tool tips on the task manager. Those are actually the least annoying to me. I found the Show Informational Tips check box in System Settings > Workspace Behavior > Workspace. Unchecking the check box does not turn off the most annoying tool tips for me. Ironically, System Settings have huge tool tips on the icons. It would be great if there were an apparent time setting for tool tips instead. One could set the timing from zero to infinity. The default could be three to five seconds. If one could not read the whole tool tip, move the mouse over the tool or widget again.

Additionally, under System Settings > Workspace Appearance and Behavior > Window Decorations, click the button that says Configure Buttons. The check box says, "Show window button tooltips." This is an additional all on or off approach for window control buttons only--for example, close, minimize and pin.

@thomassisson: Please state which tooltips are the most annoying, and where they can be found.
With regards to System Settings - you can disable the enhanced tooltips in the application settings.

The separate settings are present in this case because different applications provide these tooltips - Plasma providing some, KWin others, and individual applications and applets provide their own as well. Plasma Applets should obey the global Plasma settings however.