They are silent comments. People who have nothing to add to the discussion but want to show they are thinking about the events. I guess it is a shortened form of "R.I.P." for when that doesn't really cover it. MetaFilter is one of the few places on the web where you can safely read the comments, because even discussions happen in a civilised and often link-sourced way. My favourite place to follow events such as this, if there is such a thing as favourite when something this horrible happens.