Portrait when I'm surfing the Net or checking my e-mails. If I'm holding the iPad then it's portrait but if it's docked and I'm using the keyboard I have it on landscape.
Landscape 75% of the time. Exceptions: Words With Friends (works in portrait only), reading eBooks (more comfortable in portrait), playing Ancient Frog and sometimes Twitter (using TwitRocker or Tweetings).
Landscape for me. I love iBooks since I can read in landscape from a screen that looks like an open book. I have actually found myself reaching to physically turn a page, forgetting I am on the iPad!