I'm currently working on a project that involves a arduino(freeduino) with a extra ATmega328P on a shield. They serial communicate trough the TX & RX.
As you may know or not know, to combine/connect two arduino's you have to cross those two. Tx connected with RX and RX with TX.
But you can see TX as a output or as a input. Here you can find a good discussion about it.http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1270994948
As I was working on my project I clarified that I cross connected them. The TX of my arduino board went to the RX of the ATmega328P chip. And the same for RX.
But did it worked? Nope. Since I have a Freeduino and they just labeled TX and RX the opposite, it didn't worked. It took me too long to find this problem. And it ain't funny when you just soldered 3 hours of smd parts by hand, just to find out that there are two lines wrong. And yes I breadboarded everything and it worked well. Luckily there was enough space to solder some two bridge wires on to the board.
Hope you don't make the same mistake, thats why I post this threat.