[If there is a situation where there is a huge pile of transactions backing up, perhaps the ones with fees could get move to the front of the pack, so to say, but the rest would generally be cleared in subsequent blocks.
Such a priority ranking system is in the current release client.
I presume this has been used in the test network, but are there any blocks where that has been necessary in the regular client? I'm not asking somebody to deliberately force enough messages to be processed to do that, but more as a general question, is it something which has been done?
Yes, I realize that it is in the current release.