This is effectively a $5,000,000 reward for breaking elliptic-curve cryptography: http://blockexplorer.com/address/1PZjfkLZBT7Q3UFmyEWH8QtuzTMi3MUiBj  The public key for this address is in the blockchain.

Bitcoin's security also rests on SHA256's security, but so far there have been no useful attacks.  I think the cryptography and blockchain rules are very robust.  What is maybe less robust is the Bitcoin software and network protocol.  If we have any problems, it will be because of software/protocol problems (buffer overflows, DDos, poison nodes, etc.), not problems with cryptography or rules of the Bitcoin blockchain.

When I have some time, I'm going to work on auditing the Bitcoin source.  If you'd like to support me, donate here: 1uWY36pcXhLC4V4pZ2eZFScRaUPXgPEwC