I have encountered at least one person and read from one book, that Silverlight 3 does not provide a visual designer in Visual Studio 2008. Silverlight 2 had a preview only "designer", that would show the changes you made to the XAML in real time. The person/book suggested this would have been gone with Silverlight 3.
This is of course not true. Although a carefully hidden feature, it is very easy to enable it. After that you are not required to build and run your Silverlight application to see a preview of it.
Here is what VS 2008 looks like after you installed the Silverlight 3 tools:
I would like to call your attention to the very bottom, where you can see double lines. I drew a red rectangle there in the image above. When you move your mouse very carefully there, it will switch into a splitter cursor. Sometimes it will just flash for a second, but move your mouse very gently, and you can find it there (it is probably only 1 pixel thick, so accelerated mouse might have problem finding it).
Now click and drag, and voila!
Now simply click resume loading the designer. And you now have your full XAML preview only designer. You can zoom in and out and check out how your XAML page/control will look like.
While you cannot use it to actually drag and drop controls, it still saves a lot of time when you do not need to compile and run to see the end results.