Just read the spec document, was a pleasure to read. Wish more devs (and mathematicians!) would write as clearly and explain motivations like you do.
Some thoughts about the op codes:
1) Would it be useful to have a pause code and a resume code? Maybe for event handling, e.g. onAMReceived() { resume; } ? (Is running out funds the only way to pause a running script?)
2) A note for James: subleq isn't exactly 0x89 SUB_LEQ, it's actually
@addr1 -= $addr2, if $addr1 <= 0 then
pc = addr3, rather than pc += $addr3 . James, you'll need to compose it with additional instructions to have it match up with the subleq used in the example C++ libraries.
(This must be Ian's plan to slow subleq even further

)