ExcaliburDude - NOOOO! CUBS LOST! says: Dude, that's your 'freaking' problem. :P
1. Somewhat sheepish (but good-humored) acknowledgment of (usually one's own) silliness, foolishness, absentmindedness, or ineptness. While often used to reflect one's own state, it is also used in reference to others or situations.
2. Mild disgust, in other words, a stand-in for "ew" "yuck". (More profound revulsion can be expressed with "XP", which is similarly meant to depict a hanging tongue, but adds eyes squinted shut or nearly shut)
3. Tongue sticking out tauntingly. (Less common. As said earlier, :P is usually is interpreted as a tongue *hanging* out, not protruding straight forward)
This emoticon is a less literal depiction of expression than some other emoticons. For instance, someone who says in an IM conversation "I totally flunked the history test today :P" would not be likely to actually let his or her tongue actually loll out thus if the same conversation was held face-to-face. It probably has its roots in long standing cartoon practice of showing stupid/knocked-out/dazed people with their tongues limply hanging out like a dog's tongue on a hot day.
The P is often capitalized, but not always. Additionally, there are variants forms like :-P, =P, :b, :-b, :-Þ, :Þ, etc.
2. "Ugh, that fat, hairy, naked guy bicycled past my house again today. :P"
3. "I got the lead role in the play, so ha! :P"
Person 2: That made ma chuckle :-P
Person 1: Just a chuckle? I was LOLing my ass off.
A: My brother is dying! :P
A: Just kidding. :P
A: I'm bored wanna go out somewhere? :P