The simulated reflection effect in WPF (formerly Avalon) is ripe to quickly become a visual cliche, much like the aztec temple-like overuse of stacked 3d elements in the VB 3.0 Win3.x days. When done tastefully by someone with an eye for design, it can look good. In the hands of average Joe, though, things can quickly get ugly.

When I did the Sparkle hands-on-lab at the PDC, the reflection effect was part of the lab. One way it can be done is to use a visual brush to paint a copy of the other UI elements below, and then apply an alpha gradient to it so that it fades into the background surface.

I suppose true ray tracing would be just a little too costly ;-)

There are bound to be a bunch of these types of UX issues in the first few generations of WPF-based applications. The best thing to keep in mind is just because you can do it doesn't mean you should do it. All things in moderation.