首先你需要下载Microsoft Web Platform Installer 2.0 Beta
下载完后启动安装包，它会去获取最新的网络安装包信息
我惊喜的发现Silverlight 3 Tools for Visual Studio 2008 SP1 Beta已经可以下载了 
其详细信息如下图所示，并和Silverlight 2 Tools比较，发现Silverlight 3 Tools的大小反而小了很多，压缩了1倍多
Silverlight 3 Tools的版本号由Silverlight 2 Tools的9.0.30729.146提升到了9.0.30729.463 
此外我们还从上面看到了ASP.NET MVC 1.0正式版
这两个Web组件估计都是明天在MIX2009上才发布的
不过不小心通过Microsoft Web Platform Installer 2.0 Beta泄露出来了 
但是目前的问题是简体中文版的VS2008 SP1不能安装（亲身测试过）
Silverlight 3的文档也已经提供了下载
Silverlight 3 Tools For Visual Studio Beta也可以从这里下载
在英文版的VS2008 sp1上安装过后，Silverlight 3 Runtime版本号是3.0.40307.0 
Silverlight 3包含了三个工程Silverlight Application，Silverlight Navigation Application（这个是最新的），Silverlight Class Library 
创建网站的类型依托现在依托在ASP.NET网站 
ToolBox的控件也增加了很多，包括所有的2含有的控件+Silverlight Toolkit包+少量新控件（比如ErrorSummary等） 
Silverlight 3 Beta和Silverlight 2之间的兼容性做的不错，看下下面的 Breaking Change文档就知道了
Ensuring That Your Silverlight 2 Applications Work with the Silverlight 3 Beta ReleaseIntroductionThis topic discusses the changes made to the Silverlight runtime and Silverlight tools between the Microsoft Silverlight 2 and Silverlight 3 Beta. The changes discussed in this article are focused on changes that might cause your older Silverlight-based applications to now fail or behave differently, not on new features/enhancements for this release.
Note Corrections/Additions to this document (if any) are listed here.
Contents:
· OpenFileDialog.ShowDialog() can now only be opened from a user initiated event
· HyperlinkButton now navigates the entire window and not just the IFRAME that Silverlight is in
· Mouse events inside a Popup now give positions relative to the pop-up not the entire application
· ReadOnlyObservableCollection moved from System.Windows.Controls.Data.dll to System.Windows.dll
OpenFileDialog.ShowDialog() can now only be opened from a user initiated eventWho Is Affected: All applications.
Summary
For security reasons, OpenFileDialog.ShowDialog() may only be called during a user initiated event (MouseLeftButtonDown/Up and KeyDown/Up) or derived user event like clicking a button, checking a checkbox, etc. This brings Silverlight into line with how the other major browsers and plug-ins behave.
HyperlinkButton now navigates the entire window and not just the IFRAME that Silverlight is inWho Is Affected: Silverlight 2 applications recompiled for Silverlight 3 Beta.
Summary
In Silverlight 2, HyperlinkButton behaved inconsistently across browsers. In Internet Explorer, HyperlinkButton will navigate the entire browser not just the current IFrame.
In other browsers, HyperlinkButton navigates only the current IFrame, which is consistent with HTML behavior. In Silverlight 3 Beta, we've changed the behavior inside Internet Explorer to match the behavior in other browsers.
Mouse events inside a Popup now give positions relative to the pop-up not the entire applicationWho Is Affected: Silverlight 2 applications recompiled for Silverlight 3 Beta.
Summary
Suppose you have an element inside a pop-up that listens to mouse events:
[xaml]
<Popup>
<Border Name="border" MouseLeftButtonDown="OnMouseLeftButtonDown"/>
</Popup>
[c#]
void OnMouseLeftButtonDown(object sender, MouseButtonEventArgs e)
{
… e.GetPosition(border) ...
}
In Silverlight 2, the coordinates from MouseButtonEventArgs.GetPosition will be relative to the entire Silverlight plug-in, even though the application asked for coordinates relative to the border control (the parameter to GetPosition is supposed to indicate the frame of reference). This has been fixed in Silverlight 3 Beta.
ReadOnlyObservableCollection moved from System.Windows.Controls.Data.dll to System.Windows.dllWho Is Affected: Silverlight 2 applications recompiled for Silverlight 3 Beta.
Summary
This class has been moved into System.Windows.dll so that other APIs in System.Windows.dll can use it. In order to minimize download size impact we've removed some uncommonly used methods.
Note that this will not affect applications compiled for Silverlight 2. If you use ReadOnlyObservableCollection in Silverlight 2, your .xap file will contain a copy of System.Windows.Controls.Data.dll. When Silverlight 3 Beta runs your .xap, it will use the .xap's copy of System.Windows.Controls.Data.dll and that version of ReadOnlyObservableCollection, not the version that ships in the System.Windows.dll of Silverlight 3 Beta.
(转自互联网)