Neither. I see attrition for a number of other reasons

(i) People who realize their technologies won't be as key as they once were being touted. Indigo and Avalon have gone from "pillars of Longhorn" to add-on libraries written in .NET. WinFS isn't scheduled to impact the client OS until Blackcomb when for a while it was supposedly the most key feature of Longhorn.

(ii) People who thought Orcas would be WinFX not Whidbey. Some of the rush of working on the .NET Framework was the thought that you were going to be building the next version of Win32. Well, now we've found out that that isn't the case and instead what we have done in Whidbey is most likely what will form the basis of WinFX. Working on v2 is never as sexy as working on v1.

(iii) WinFS folks who don't want to wait 3 or 4 years before they ship.

The interesting question is which people at Microsoft fall into any of the above 3 categories. Heck, I've asked myself if I fall into any of the above categories recently.

There's a fourth group of people I also expect to be moving over time as well but they're issues are tangentially related to the Longhorn changes

(iv) People who came to Microsoft to "change the world" but became frustrated with the inefficiencies of working in a company the size of Microsoft such as the bureaucracy and internal politics.

There are probably some people who it just slowly dawned on that they'll never get rich working at Microsoft. These folks would also be interested in moving but I consider the number of such people to be small enough to be negligible.