As most of you know, one of the best ways to keep some rude person from pushing you (or shooting you with a push bullet) is to sit on an object. But, WHY does sitting give you this protection?
The answer is, your avatar becomes phantom when you sit on something. Other avatars and objects pass right through you. This is actually a pretty good thing, for more reasons than defense against griefers. It would be hard to dance with a woman in a full skirt if you kept bumping into it! (Sitting on a poseball is still "sitting", even if your avatar is waltzing with a partner.)
Those of you who use the Phoenix and Firestorm viewers, and possibly other third party viewers as well, may be familiar with the "Phantom Mode" feature they offer. By hitting CTRL+ALT+P, your avatar becomes phantom, just as if you were sitting on a prim. That is, it DID become phantom until the latest round of server updates from Linden Lab.
This update removed the functionality that permitted the Phantom Mode to work. Oh, your viewer still says "Phantom Mode On" as if it knew what it was doing...but it's NOT on, and people can still shove you around.
I sure hope Pathfinding and forced attachments and teleports are worth it.