software patents threaten open source

In a recent interview with the BBC Bruce Perens expresses concern over 
the way software patents could limit software development to only large 
corporations.  He says *" We're looking at a future where only the very 
largest companies will be able to implement software, and it will 
technically be illegal for other people to do so"*

I guess what I really want to know about the issue of software patents 
jeopardizing the future of the open source software movement is if a 
software, such as Linux, was patented but not enforced, could there 
continue to be contributions from anyone, and could it still be 
customized for personal use?

And is the patent process so expensive and so arduous that a small 
software developer could not easily get a patent?  What would make 
software patents "technically illegal?" The idea of having to consider 
protecting software with patents seems to be anti-open source.