Consider me to be uninitiated - is this relevant only if I'm a miner, or if I'm running any full node?
Pfft. If you were a miner 99% chance you've never seen a full node in your life. Any full node can call that rpc, and only full nodes can call that rpc.
Ok, understood. And if this call indicates a positive response for a given transaction, how much overall confidence does it give you about inclusion likelihood?
Anyone know about this?
I'd really like to some kind of metric about block inclusion likelihood for a given transaction.
A lot depends on the priority of the transaction, number of transactions queued etc. Generally, if you were to calculate the priority for each transaction, that would give you a good metric showing which ones were most likely to be included sooner than others. e.g. you could say, transaction A is most likely to be included while transaction Z is least likely. I am not sure if there are statistics about it.
It could be calculated if you were to run a full node, calculate the priority when you see a transaction, and then see how long it takes a transaction of a particular priority to be included in a block. Doing that for a while would let you create statistics - e.g. perhaps by 20% baskets and then you could say, the top 20% are included within X time with a confidence interval of Y. Then for each of the other baskets.
e.g. Look for priority calculations
https://en.bitcoin.it/wiki/Transaction_fees
