An abbreviation for "in my opinion", most commonly used in text messages or Twitter updates when space is limited.
Can also be used in every day conversation, though is often intentionally condescending or laced with attitude in that context.
In a text message: "IPO, we should go to Mike's Bar- they have the best burgers, and I'm hungry."
In every day conversation: "Excuse me, but I won't do that. IPO, it sounds like a horrible idea. Let me know what it's like to narrowly evade arrest."