I've seen people refer to this, but I haven't seen it posted yet:

http://spoilertv.blogspot.com/2009/05/big-bang-theory-episode-223-monopolar.html

(I have no idea whether it is watchable outside the US)

I love Penny and Sheldon's interaction there. :) I can think of other places I'd like to lock the two of them. ;)

And doesn't it seem like it was Sheldon who told Penny about their trip -- before Leonard thought to?