This comment thread made me realize that there is still a lot of confusion about what the different RIA technologies do. Statements like "Flex is Adobe's AJAX offering." or "Flex is like a weak version of WPF/E when it comes down to it." just show people haven't quite grasped where all of these technologies fit. Hopefully this can shed some light.
OpenLaszlo - An open source Rich Internet Application technology. The IDE is Eclipse-based, you write a combination of XML and ECMA Script (called LZX) to build the UI and add functionality. With OpenLaszlo 4, you can deploy OpenLaszlo applications as either Flash apps or DHTML/Ajax apps.
Flex 2 - An application framework that enables developers to easily write apps on the Flash platform. Does not require a server, uses an Eclipse plugin as the IDE and requires Flash Player 9 for the user to run apps. The XML user interface language is called MXML and the scripting language is ActionScript, an ECMA standard. Good solution for building applications entirely in Flash. Flash Player 9 is currently available for Mac, Windows and Linux.
Windows Presentation Foundation (WPF) - Part of the new version of the .NET Framework. The IDE is a combination of Visual Studio (the next version, "Orcas" has integrated support for WPF) and Expression Studio and the UI language is XAML. WPF Supports native 3D acceleration and can be deployed as a browser application called XBAP. Meant for giving developers the ability to build great looking desktop applications. Currently only runs on Windows Vista and Windows XP.