If you want to perform custom animations in code that you can't do using XAML, you need timers. In the full blown WPF you have several options, e.g., System.Threading.Timer, System.Timers.Timer and System.Windows.Forms.Timer.
You normally provide a callback that gets called when the timer elapses from a background thread. Properties on WPF objects can only be set from the foreground thread, so you have to queue a call on the UI thread to perform the actual animation. You can do that by calling the Invoke or BeginInvoke method on the System.Windows.Threading.Dispatcher class. You can access the correct Dispatcher instance to use through the Dispatcher property on the UI element (*).
Another option in WPF is to use the Rendering event of a CompositionTarget instance. In that case you get called when WPF is ready to render a frame. The frame rate depends on CPU speed, GPU performance, graphics complexity and other factors, so it fluctuates. This means that the interval after which you get called also fluctuates. However this is great for some scenarios.
In the current Silverlight 1.1 alpha your options are more limited. The CoreCLR libraries do have a System.Threading.Timer, but there is no Dispatcher class to delegate work to the UI thread. So it is useless for doing custom animation. In the source of the Monotone sample by Lutz Roeder I found there is an HtmlTimer class in Silverlight 1.1. This class is undocumented and marked obsolete. Visual Studio shows a warning after compilation:
'System.Windows.Browser.HtmlTimer' is obsolete: 'This is not a high resolution timer and is not suitable for short-interval animations. A new timer type will be available in a future release.
Lutz shows how to use an HtmlTimer in his sample. HtmlTimer has a Tick event. Any event handler that you wire-up to that event gets called from the UI thread. So that solves the problem for the time being.
When I tried to Google for more info on HtmlTimer, all I found was this blog post by Mike Taulty which mentions this class in passing.
(*) In fact any .NET class that derives from DispatcherObject has a Dispatcher property.