A couple of weeks on, I can say that my major uses of the ipad are:
1. Keeping me awake when I should just go to sleep. It's even better than the iphone for that because the battery is bigger and its more fun browsing. I have had to ban both iPhone and ipad from the bedroom because of this.
2. Going to the pub while pretending to work, and actually getting some work done. Without the annoyance of having to boot up the laptop, lug it there in the first place, dial in with the dongle, make sure its asleep when go to the loo or lock it to the table etc. I can sling it in my bag, maybe with the Bluetooth keyboard, and it's small and light; just whip it out, turn on and I can write notes, outlines and documentation, plan tasks, clear my inbox etc. The 3G implementation is completely transparent as opposed to laptops and I'm much less restricted in the space I need to use it. I'm much more prepared to carry this around than a laptop which means I'm much more likely to have it when I come up with a good idea.
Of course none of this is specific to the iPad - it just happens to be the only real mobile tablet around at the moment.
I confess to still being more likely to use a fountain pen and paper notebook to write and diagram and so on but one can have both. Paper won't sync to the desktop - well, it will, by the conduit known as "typing it up".