I have a feeling MS and IBM are in a state of MAD when it comes to patents. One reason IBM has crushed SCO so publicly and thoroughly is to show anyone - especially MS - considering a patent war with IBM what they are in for.
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.