In order to enable interoperability we need to have a sufficiently detailed specification of the scripting language, how mathematically scriptSig satisfies scriptPubKey and some commitment from Satoshi that he will "play nicely" with such efforts.
I think it is WAY too early to nail all that down; bugs in that code caused changes just a few months ago.
Just write the code, and when core bitcoin changes, be prepared to change your code. For the specific case we're talking about, you don't need to know all the possible ways scriptSig satisfies scriptPubKey-- you just need to reverse engineer how the standard send-to-bitcoin-address works so you can sign your own send-to-bitcoin-address transactions.