With this release, Microsoft joins, albeit late, a crowded market for Ajax related frameworks provided by open source projects, small or large (Google) companies. Market is perhaps not the right word as a vast majority of solutions are available for free. Microsoft product will certainly get some following in the .NET community and this will hurt current providers of paid frameworks like Telerik or zumiPage. With its free client-side library, Microsoft can also appeal to people looking for developers outside the ASP world.
More information on the Web
Ajax: A New Approach to Web Applications (original Ajax article)
Wikipedia on Ajax
Wikipedia on Ajax frameworks
Comparison of AJAX frameworks for ASP.NET
Exploring Ajax Runtime Offerings (java oriented)