I'm wondering why it isn't called Microsoft Communication Foundation. Does it rely on Windows and will it rely on Windows in the foreseeable future?
Windows Communication Foundation, Windows Workflow Foundation and Windows Presentation Foundation all require the Windows OS.
WCF and WWF services usually run on a Windows Server, and WPF is also windows-only.
Just marketing, I remember around when .Net was initially released, they rebranded all the servers (Exchange, Sql Server etc) to be called the ".Net Servers".
I believe it's part of a larger branding strategy by Microsoft. For example, Windows Workflow Foundation and Windows Presentation Foundation are similarly not really tied to "Windows" as such, either.
(You could probably argue that WPF is "more" tied to Windows than the others, but Silverlight shows that much of WPF can be run on other platforms as well)