Does SPF break email forwarding?

Yes, it does. You'll have to switch from forwarding, where the envelope sender is preserved, to remailing, where the envelope sender is changed.

This is a rather serious issue as internet forwarding is very common and remailing is not trivial. The guys at pobox.com say "You can't make an omelet without breaking eggs, and unfortunately forwarding is the egg that breaks. We're doing our best to patch it back together with SRS". We wish them good luck because withour these patches SPF is rather useless.

You can get more information, including some work arounds at the SFP home page:

spf.pobox.com/faq.html