On JavaZone (which was an huge success, IMHO), Richard Stallman was the guest of honor. He talked about the four freedoms which free software gives you, how the Sun Java implementation is not free, and why that might concern us. Then, he talked about something that should concern all of us, even if you think Free Software is bogus: software patents.
As I understand Stallman, if software patents become law, you can violate a patent without knowing it. Also, based on an inspection of source code, he made a back-of-the-envelope estimate that Linux contained somewhere between 10,000 and 60,000 patent violations. This means that your program probably contains thousands of patent violations.
This means that any program is succeptible to patent lawsuits, and any patent-holding organisation has a way of attacking corporate rivals. This means that in the future the number of lawers in a software development organisation will exceed the number of programmers.
If this is a future you don’t want, you should oppose software patents.