Derik Whittaker shared his experience with using Silverlight Control Toolkit for Silverlight 3 with Windows Phone 7.
As we get closer to the launch of WP7 (Windows Phone 7) more and more developers are starting to play around with the SDK and build out applications. Since WP7 supports Silverlight 3+ (3+ means it is basically Silverlight 3 w/ some tweaks to work in a multi-touch environment) it is logical to think that you should be able to use the Silverlight Control Toolkit for Silverlight 3.
However if you simply take the binaries from the source on codeplex and try to use them with say the expander control you are going to get runtime errors. The errors revolve around the GlobalCalendar, but they are very easy to fix (assuming you do not need the global calendar control).