OK. I was a little slow on the uptake on this one. But really..

Why didn't Peter just leave Nathan a note. Something like.

Hey... I'm still alive. A friend of mine healed you with his blood.

IIRC, they didn't really have the time to do much of anything other than have Adam give Nathan some of his blood, then run out of there since they knew they were being tracked. As always, hindsight is 20/20.

They had plenty of time to grab some plane tickets and get a passport. I'm sure they could of slipped a note in there.

that would have been an interesting note.

"Hey Nate! Hope you like the not ugly face!

Love, Pete"

Seriously, I think he would have wanted to be a little more personal then a note.

Sure and leave Nathan drinking and in the dumps while he runs around and saves the world. Great idea! Nice and personal.

You're forgetting that Peter didn't exactly anticipate getting his memory wiped and locked in a crate bound for Ireland. Why leave a note when Peter more than likely saw no reason why he wouldn't be able to get some form of contact out to Nathan after he and Adam got the hell out of New York.

I'm sure that writing a note while waiting in line for a plane ticket was something that'd be too hard to do.

I think Peter might have been hoping to talk to Nathan, and just didn't think to write a note, just in case.