Twitter, like any social media, is about forming communities and relationships. During SxSW, I had more people "recognizing" me in person because they follow me on Twitter.
The phrase "Tweet Up" was coined within the past 6 months, and refers to a group of Twitter users getting together. Local "tweeps" get together in real life (which Twitter has taught me to shorten to IRL) and... hang out. There's definitely nothing formal about these events.
Vancouver is having it's first Tweet Up on Thursday, April 17th at UVA Wine Bar on the corner of Smithe and Seymour. We're getting underway at about 6 pm and have 17 people showing up so far.
If you're interested, come on by. We'll be the loud boisterous group of nerds in the back of the room.