The announced battery life for a iPod is usually way under what you'll get, assuming that you use it as the average person would, and that means not letting it play for 12 hours straight, but instead switching songs rather often, and browsing your songs.
Personally, I browse my songs a lot, and I know for a fact that the battery bars on my iPod go down heavily when I do that. Sure, they then stabilize, and even go up, but that at least shows that simply browsing through your songs is heavy battery consumption.
Same probably goes for the iPad. Pretending that it will get any specific time of battery life, regardless of the activity, is brainless at best, and a canard at worse. Consistently watching movies with all sorts of options turned on will inevitably shorten the battery life.
Also, what happens when the battery dies on that thing? Obviously I guess you'll have to send it to Apple, for a fee, and get an all new one (or at least a refurbished one). Another downside for the thing...
Personally, I don't see why I'd need to fill the gap between the mp3 player and the laptop. What's likely to happen is that it will try do the work of both, worse than the laptop and mp3 player. You want your mp3 player to be portable, so the iPad obviously doesn't leave the start-box in the race against the mp3 player. As for the laptop, you want it, among other things, to be powerful, convenient, widescreen, long lasting and versatile, none of which I think the iPad will be.