Google just rolled out a new version of Google Mobile Maps today that includes traffic overlays. I live in Los Angeles, and hence this application just went from “its pretty cool” to “its damn near imperative that I have this with me at all times.” The UI is pretty cool, although I wish it had speeds. It overlays the freeways with red/yellow/green lines to indicate flow speed. It even works with the off-ramps and interchanges.
You can download the service by going to http://google.com/gmm on your Blackberry or most other J2ME capable phones (but its prettier on the 8700).