After a couple of months of this I think the verdict is: sometimes.
Let me explain, many people have said that the iPad is a device ideally suited to consuming content rather than creating it. So for books, browsing, video and so on it really is quite good (mind you I still think the e-ink displays on readers like the kindle and iRiver Story beat the iPad for books).
Interestingly enough the iPad keyboard isn't the problem when creating content (though it does take up a big chunk of the screen), the problem is multi-tasking. When using a laptop I'm often referring to a web page while writing a document or editing a spreadsheet - the iPad just doesn't let you switch between applications smoothly enough, in fact it comes off as a kind of not-very-good-netbook when used in this way. I end up using the iPhone as a surrogate second screen, with the iPad as the editing tool and the iPhone my view of the reference material.
While on the subject of the iPhone, will apple please hurry up and fix that proximity sensor problem - having the (otherwise great) phone randomly switch to handsfree/mute/hold/facetime when on a call is very annoying.
Still, so long as you don't need to crunch big spreadsheets the portability and range of apps for the iPad mean it is a great travelling tool, I'm really liking mighty meeting for showing PowerPoint presentations and documents to go integrates well with our favourite document sharing system, dropbox. So I don't think I'll be leaving the laptop behind on multi-day trips, but when I'm going out for a few hours I think it'll be the iPad that I'm reaching for on the way out of the office. Hmm, perhaps I should pick up a new laptop bag from our new client for those days...