Hi Joey,
For Blend 3, it will be even better than that. Not only will you be able to specify d:DataContext to simulate a DataContext at design time, but you will also be able to specify your own sample file that allows you to use custom types so data shows up on the design surface. Stay tuned for more details - I will blog about this as soon as we ship Blend 3 over here -
http://blogs.msdn.com/unnir (we currently only have the designer focused sample data feature in the preview build of Blend 3).
This is the developer-enables-designer-focused addition to the Blend sample data functionality, in addition to what we already have in the Blend 3 Preview that you can use to quickly mock up arbitrary data sets.
Thanks,
Unni