No, it's not just FUD, and it doesn't just apply to LINQ either. Microsoft only promised not to sue over certain kinds of patent infringement in core libraries; that promise didn't extend to code you wrote on top of those libraries, which is unsurprising. Unfortunately they've patented a bunch of common ways of using .NET functionality, including many of the things you might use LINQ for and one of the most common idioms for using delegagtes and events. (Both of the commonly used GUI libraries - Windows.Forms and Gtk# - infringe the latter patent. So if you're running a GUI-based .NET app on anything but Windows then Microsoft can sue you if they ever decide it's to their advantage to do so.)