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.)
If Microsoft is willing to spend millions of dollars and take a huge PR hit to sue you, it's safe to assume that whatever you've built is successful/popular enough to make you very a wealthy man.
Possibly it matters if it's not you that's sued, but you have similar usage as the company that is. Investors may or may not find that an acceptable risk if MegaCorp XYZ is currently embroiled in a large lawsuit.
For example, if your company had the same strategy as Google (and/or made some similar mistakes) with regard to circumventing Java licensing/restrictions, I could see investors or possible investors that were aware of that being somewhat spooked at certain points in the past.
