You can still detect a double click using a mouse up event handler and a timestamp - but should you? The omission of a double-click event wasn't because of a technical limitation – the omission is a deliberate design decision. Silverlight is built for the web, and many consider double-clicking on the web to be a usability no-no.
Here is what Peter Chng has to say:
I'll admit that while I was first confused by this action, I thought little of that user's comment - I mean, how hard can it be to learn a simple action like that? But, after some more thinking, I've come to agree with the comment - breaking the pattern of how a user navigates on a website is not a good idea, even if it's done to try to make your website feel more like a desktop application.
The New York Times website includes a feature that launches a search (in a pop-up window) when you double click a word inside an article. Digital Inspiration believes this behavior is user unfriendly, while Dion thinks the feature is a bug.
You might also have noticed the only mouse button events in Silverlight are LEFT mouse buttons events. Edit (I haven't kept up with changes in the other lifestyle...)
Macs don't have a right mouse button, and my guess is they never will (adding one now would be like admitting defeat). Exposing a right-click event could only invite more usability problems.
Silverlight has some wonderful potential. Just remember to innovate – don't aggravate.