G and P more or less have the same frequency, so it doesn't matter either way. Nevertheless, I've contemplated on the issue for a long time before I made my decision.

There are a few reasons that I've decided to keep them that way:
*  In literature G tends to be more frequent than P, but in other types of text, it's not always the case. According this, in the Brown Corpus and the British National Corpus, P is actually more frequent than G. I think the source of the article is: Eckler, A. Ross, Word Ways, "A Survey of Letter-Frequencies" , p. 75-81, but I wasn't able to verify that. If someone has access to the Brown Corpus or the British National Corpus (which are huge and contain many different types of text, and not just literature), it would be nice to independently verify that.
* Even though the distance to hit the G key is greater, hitting the P key is a bit awkward, because it involves curving back the index finger.
* G hasn't been moved horizontally (like B and H that have been kept in place) to mark on each row the edge between the left hand and the right hand to help in learning the keyboard layout. It slightly helps to mentally separate the right hand keys and the left hand keys for those people coming from QWERTY.
* As a bonus, P keeps the same position as on the Dvorak keyboard layout. Only the A, M, P and the number keys are the same on Dvorak and Colemak.
* G is most of the time preceded and followed with a character from the right hand, so even though it takes slightly longer to reach the G key, there is usually more time to prepare for it.
* But the main consideration was that the PT/TP same-finger digraph is much more comfortable the way it is on Colemak, than if P and G were to be switched. The GT/TG digraph is much less frequent.

U is a bit more simple. The index finger needs to hit 6 keys, and putting the U there with 5 other keys would increase the same-finger way too much.  If you'll mix the U with 5 other consonants it will make the same-finger really high; and you can't put other vowels there because all the other vowels need to be in the home position.

I second that.  I love these posts and simply collecting them to serve as the base of a future comprehensive article on Colemak is a great idea!

There should be a section of FAQ* dedicated to why the Colemak is what it is, in detail. I'd also be interested to know how Shai knows so much about keyboards. I've been researching keyboarding for 4 months, but Shai seems to know so much more than me.

As to a couple other things I've noticed:

-B is in a very awkward position, and although it's not very common, Q or Z would do better there. I do know that leaving it would help people learn Colemak, but since that isn't a problem for me, I'd probably switch it with X. Actually, I might do that.

-L is under the index, but I think top row ring finger is actually easier to type. Of course, switching Y with L would cause same finger issues.

*FAQ is hard to type on Colemak, though I was using the left shift for the whole word, which didn't help