Silverlight 2.0 just went live. If you are a .NET developer building ASP.NET or WinForms/WPF applications this is a HUGE deal. Your .NET code can now run within a browser and across platforms (including Mac and on Linux using Moonlight) without requiring the .NET framework installed.
I have never spent much time learning AJAX but I see XAML/WPF basing used heavily in the future. When Silverlight 1.0 was first released I was excited at the prospect of re-using WPF knowledge to build web applications but was sorely disappointed with the lack of tooling and controls. Fast forward today and you have rich tooling support in Visual Studio a number of control (including a grid view and date picker) plus more controls being released by the Client Controls team.
Start learning Silverlight today your going to need it soon. Goodbye AJAX I will avoid you whenever I can.
To get started on Silverlight see the great ScottGu.