FYI Microsoft has a cross-patent licensing agreement with IBM and has for many years, which is not unusual for companies of that size. Novell recently entered such an agreement as well, but so far they are the only free software vendor I'm aware of.
I don't think those agreements cover linux, which IBM is heavily invested in and MS has made noises about. I'm guessing IBM doesn't wish to find itself in the same position as Vonage/Verizon or RIM/NTP over any potential infringements in linux.