Be honest with a genuine developmental area you need to work on, or a skill you already have but want to deepen. The ole "I am TOO dedicated to my work" is sooooo trite and every interviewer worth his or her salt will immediately roll their eyes.
Examples:
* public speaking
* deepening your existing skills as a supervisor/"coach" to others
* time management
* tend to be detail-oriented but need to always remember to step back to see the big picture
* embracing the 80/20 rule (80/20=focus 80 percent of your time and energy on the 20 percent of your work that is really important. Don't just "work smart", work smart on the right things)
Here's a link to other hints and examples. Google more if you wish:
http://askamanager.blogspot.com/2007/08/how-to-answer-what-are-your-weaknesses.html
This is a great example. When I was in HR and Executive Search... I got so many "I'm too dedicated to my work". What's almost just as bad but works is "I'm a perfectionist and sometimes I spend too much time trying to get things done to my expectations... lalala".
The examples that Redney wrote down are great... because to an interviewer they may not be seen as a "weakness". Someone who thinks they are not the best at public speaking or time management can always develop. And those are definitely not as bad as someone who says "my weakness is showing up to work late 3 times a week and wearing my pj's".