QuirksBlog: CSS vendor prefixes considered harmful.
See also: Not Supported by Jonathan Snook.
It’s issues like these that have me concerned about whether or not we’re ready for CSS3. Sure, writing a few extra styles isn’t really that big of a deal, right? Yes and no. We’re coming into an age that is accepting a more radical implementation of CSS every day. With that comes even more variance as to how properties (and their values) are implemented. That could very well result in an endless combination of vendor prefixes as well as values that create a major headache for the people actually working with the styles.
PPK goes into quite a bit of detail pleading his case. I urge you to take note.