System.Collections classes are obsolete
Collection classes like ArrayList, SortedList, Stack and Hashtable will shortly be obsoleted (i.e. the ObsoleteAttribute will be applied to them in upcoming .NET Framework builds). You can start to see this in the Silverlight Alpha (i.e. the Obsolete attribute), but they will be physically unavailable in the release version.
More details can be found in a recent Base Class Library Team blog entry Non-Generic Collections to be Removed from Silverlight.
Even if you don't plan on writing Silverlight-compatible code you should avoid using the types detailed in the BCL Teams blog because they'll be obsolete in other future versions of the .NET Framework.