It's dead simple, really: Assign address blocks to pairs of providers.
Both providers announce those blocks all the time, and assign addresses
out of those blocks to customers who multihome between those two
providers.
Subscriber A gets netblock from B who cooperates with C. Subscriber A
hates C and goes with D. What now? D has to advertize the route and C
has to stop. There goes the simplicity.
Competition.. gotta love it.
-Wayne