Multitasking and Flash are reportedly in the next iPhone OS. My guess is that the new OS isn't ready today, however, it will be when the iPad ships.
As for use, I think the iPad will make my life less cluttered and easier.
For one, I'm a heavy iPhone user. Maybe four or five hours a day. I use it more than a laptop or desktop. I use a lot of the apps and find the iPhone to fill a lot of my computing needs. A bigger version would be nice to have.
I don't readmuch on the laptop or computer - they're inconvenient. Reading at length on the iPhone is out of the question. However, if this is a good substitute for a book, I'll probably scan a ton of paper I have into PDF format and put it on the iPad. That would let me pile up stuff to read digitally without overflowing bookshelves and nightstands. For me, that's worth it right there.
Add contract-free 3G, and I can use it in many places without WiFi and in other countries. My iPhone has been useless outside the US because it's locked to AT&T.
So it makes sense for me. Maybe not for everyone, but I can justify the purchase.