On this trip I've experimented with leaving my laptop behind and just taking my iPad, and so far it's performed pretty brilliantly. So long as I have an Internet connection I can do pretty much everything I would want to do with a laptop, and even without one I can do about 80% of the things I would normally do with my MacBook Pro.
The thing that makes the difference is the keyboard. Apple makes an excellent Bluetooth keyboard, which is both extremely thin and light, and has a good solid feel: they're full-sized keys, and they have nice throw, so I don't feel like I (or my hands) are compromising. And the difference between writing with a real keyboard, and tapping on the screen, is like night and day: I can tap with more than one finger on each hand, but it's not as fast or accurate as when I'm using real keys. Not only do I make more mistakes, but I can't feel when I make mistakes, the way I do when I'm using a regular keyboard.
Of course, the other thing that makes a big difference in the functionality of the iPad + keyboard is not the device itself, but rather the fact that I've got a bunch of useful material up online that I can access when I'm writing my talks. In particular, my habit of putting pictures up on Flickr is really starting to pay off: as my photography has improved (or at least gotten more quirkily distinctive, and migrated to ever more impressive devices), that's turned into an online repository that I can access when I'm revising my talks and need to illustrate new points.
I would like to see better synchronization between my machine and iDisk, or a feature that automatically backed up files to my iDisk. Or rather, I would like this for my hosts, so they could always have access to the latest version of my talk.
I may go for an adapter to connect the iPad to a monitor, but I really liked being able to carry around the iPad and read my talk off it. I worried that it looked a little dorky, and it probable does; but apparently the aluminum back reflects the stage lights in cool ways, so I'm going to keep reading off it (and maybe look for some holographic cards or such to tape on the back for such occasions). I know it makes me look a little like Jonathan Pryce's evil Rupert Murdoch-like character in that James Bond movie, but c'est la vie.
The other thing I'll have to practice is using it as another display surface, so I can occasionally different images than what's showing in the presentation, or maybe toss our specific words than I want the audience to focus on. Not even technically or logistically difficult: I could just add pictures to the presentation text, and flip over the screen when I want to show something to people.
Though I wonder if the VGA adapter works with the iPhone? Could I do presentations in Keynote and then run them off my phone, while reading the text on the iPad? Must experiment. I should also see what the Keynote remote control is like.