This was a triumph. (Portal Thread *Spoilers*)

[Fuyudenki]

I didn't experiment enough to actually successfully trap myself(and I now have to buy my own Orange Box if I want to play further... I need to upgrade my PC, first.), but I do know, TGH, that if you attempt to place a portal on a moving surface or a surface which is too small to hold the portal, it will "fail" like trying to place a portal on a non-portalable surface.  If a surface gets portalled, and then moves, the portal will "break" and close itself.

I know this because back when Exite was released(portal mod for HL2 DM), I learned how to steal moving parts of the map and turn them into portable bunkers by sticking portals on them, so that was one of the first things I tried.

Also, note that there's no such thing as an "entrance" and "exit" portal.  They're two-way doors, so if you're shooting out of one in a bunker, the people on the other side can still shoot in.  The color difference was only so that you can differentiate between them when moving endpoints.

I saw a guy cheat his way through one of the puzzles with moving platforms over acid, by standing in the middle of the portal and then shooting the other endpoints down the hall.