"given p1.0, i may write a 1 to it so that "p1.0 pin" becomes high. if "p1.0 pin" is held low by an external signal (although i have written a 1 on it), i would read p1.0 now (from the code) as 0.

is that right?"

Yes. That is right. That being said, you should also understand why that is right. Read the tutorial and the links posted by Erik. Also do a search on this forum for "pull up resistors" and you will find a wealth of discussion on this topic.