I don't even need to log into the servers to know that the tributes being invisible sometimes is a likely bug.
I've played 1.3.1 in a few different servers already (They all used bukkit I believe, not sure about what plugins they used though.) and I've seen (Or rather not seen) people who shouldn't be invisible being invisible, which I could only fix by relogging. I also see people in Steve skins a lot when they actually have a skin on. This includes some youtube videos I've watched from youtubers who were confused why they were Steve while logged in. I'm not sure however if these two bugs are related at all.
I can only imagine how annoying it would be to die to someone invisible... I'd be so frustrated.
I also know for a fact that people being invisible is a server side thing (Or a Minecraft side thing.) because I've seen someone I know who has no hacks as invisible before. As well, my friend said he couldn't see anyone, including me, until he relogged when I could see them all just fine. (Keep in mind, this was not here on these servers. However, based on the reports in this thread of it, I'm sure it's a bug with Bukkit/Minecraft/a common plugin and possibly a problem here.)