I blogged a while ago about Artemis. This simple tool uses a blurry background to draw user’s attention onto a modal progress bar. Artemis is a good example of how you can use a blurry background. It emphasizes the importance of the modal dialog which doesn’t require any context.
You can watch a video of Artemis in action. Note that I did not take the time to polish the UI. Therefore, the blurry background does not appear progressively but instantly (and the indeterminate progress bar clearly needs some work.)
The full source code of Artemis is part of chapter 16 of Filthy Rich Clients. It will be released at the same time as the book will be available in book stores.