What about ISPs whose customers bring their own portable IP address space along with them, and then multi-home? (i.e. have two or more ISPs, and request BGP peering with both?)
The directly-connected ISPs can do their checks to make sure that their customer owns that IP address and adjust their filters accordingly... but anybody else with BGP peering to these ISPs (i.e. other ISPs) can only hope and pray that their peers are doing the right thing. Blind faith might not be good enough.
As I understand it, SBGP would implement PKI and digital signatures to ensure that only someone who actually *owns* a particular netblock/ASN can advertise a route for it.
Currently, anyone can advertise pretty much anything and it's only individual ISPs filtering settings that would prevent it getting propagated.