Source IP spoofing--why unlikely?

My http://spf.pobox.com/ suggestion and SPF question:

Move/copy section 

  "Source IP spoofing?

      Spammers may spoof entire TCP sequences to get 
  their mail out.

  Knowledgeable people consider this unlikely. If you 
  think this is a concern, you are invited to 
  demonstrate an attack."

from http://spf.pobox.com/faq.html to
http://spf.pobox.com/objections.html and expand on why
knowledgeable people consider this attack unlikely. I,
for example, am curious.

thanks,
george