Silverlight is bringing a lot of changes to VB, including cross-platform support. While much of VB is going to be available in Silverlight, including VB's collection of legacy functions and LINQ, not everything will be available moving forward.
First on the chopping block list is the My namespace. This is understandable as most of it wouldn't work in a browser context anyways.
In an unfortunate turn of events, XML literals will not be available. This won't be offered because System.Xml.Linq is not available in Silverlight 1.1.
COM support will not be available either. With a goal of being cross-platform, this was pretty much expected.
Last on the list if the File System Object. As Silverlight will only have access to System.IO.IsolatedStorage, this wasn't an option.
Note that these changes only apply to VB running under Silverlight, when using VB for compiled applications this functionality will remain.
In the Silverlight timeframe some of the base classes will be marked as obsolete. These include the non generic collection classes such as ArrayList and Hashtable.