Is it a promise to reject any block contains a different transaction with the same inputs? That might leave the miner off the real chain forever no? But otherwise what does it matter? They'll mine the tx unless another gets in first? Not much help really.
its not a promise to reject, but a promise not to mine a conflicting transaction. and to include your transaction in a mined block (so even if a block is orphaned due to other reasons, the transaction will still be included later)
so if you got a signed reply from 40-50% of the hashpower - you can be quite safe that a competing transaction won't end up in the longest blockchain.
it's not 100% safe, but it's a good indicator.
It
can be made into a promise to reject. Of course, no miner would make that promise on their own. However, if you get the initial promise not to mine conflicting transactions, you could then use that promise from 50%+ to prove to the miners that it is safe to reject, although I suspect that a decent fee would be involved in that. However, it could work to make 0 confirmation transactions completely safe after a few seconds (after you contact the miners and get enough support to have them to reject conflicting blocks)