Now that the three pillars of Longhorn are either delayed (WinFS) or available on downlevel systems (Avalon and Indigo), is there any reason to migrate to Longhorn? I think yes. (This is almost a strawman's argument.)
There's more to Longhorn than those three pillars. Aero, the Longhorn shell, is almost certainly going to be a Longhorn only, for instance. I seriously doubt the Desktop Composition Engine will be available in XP. The impact of Avalon's availability on XP will be confined to the window boundaries of Avalon applications. The difference in the user experience between Longhorn and XP will be similar to that between Windows 95 and Windows 3.1 plus Win32s (the backported Win32 dlls for 16-bit systems, if you remember). Other areas include fundamentals like security, which will be greatly enhanced in Longhorn.
From the API standpoint, there are numerous groups all over at Microsoft, outside Windows core, contributing to the WinFX managed API set, and I doubt that most of them will be bundled along with the backported Avalon redistributable for XP. A major reason is that the redistributable package needs to be kept small.
If you have the WinHEC build of Longhorn, you can get a glimpse of hundreds of the new managed API dlls offered in Longhorn by looking at C:\Windows\Microsoft.Net\Windows\*. Of those hundreds, Avalon's API is concentrated in a few dlls: PresentationCore.dll, PresentationCore2.dll, PresentationFramework.dll, UIAutomation.dll, WindowsBase.dll and perhaps a few others. (Don't look at C:\Windows\Microsoft.Net\Avalon\* directory, since those represent the discard older version of Avalon, which is still used for demos.)
Avalon for XP is likely to be a subset of the Longhorn's Avalon APIs that achieves the simple goal of allowing software developers to write code that will run Avalon code on both XP and Longhorn. I expect the Avalon to run tolerably on Windows XP, possibly requiring additional hardware, but not having the level of performance and optimization that Longhorn will offer. Longhorn, for example, supports new driver models.