Gavin Gear points out a cool sample available on the Microsoft website that demonstrates using its on the web with silverlight.
There's a new sample live on downloads.microsoft.com that I wrote a while ago that demonstrates implementing ink support in Silverlight using a reusable class called "InkCanvas" - InkCanvas code was originally written by Sam George here at Microsoft. This sample implements reusable functionality similar to the InkCanvas element in WPF.
Here's the download link.