Jason,
From everything I've read, an originization cannot announce another originizations address blocks to anyone else under ipv6.
Well, yeah. This would probably be considered theft. However, in order for the Internet to scale (IPv4 or IPv6), address aggregates must be announced -- that is, when an ISP delegates a block of address space to a customer, the ISP should not announce that address space separate from the block the customer's address space came out of. This is where the question of address ownership (o address leasing if you prefer) gets involved.
This would make getting a top level block a REQUIREMENT to multihome unless you took multiple blocks from multiple providers, but that gets messy.
I believe this is the intended strategy for multi-homing in IPv6. Rgds, -drc