The Guardian's Charles Arthur sees Apple's swift entry, re-design and eventual dominance of these markets as a pattern. The iPod introduced a hard drive-based music player to the small market and eventually took over.
Yes, the mobile phone market was huge when the iPhone was released, but the smartphone subset wasn't. Apple introduced multi-touch and apps and is working towards dominance.
Now, Charles sees the same thing happening with the iPad. I think he's spot on with this. I also think the iPad's rise to prominence will be much slower than the other devices experienced. When the iPhone was introduced, people were eager for an alternative. Existing phones were clunky, often unreliable and a hassle. Users wanted an opportunity to ditch them.
Likewise, early digital music players left a lot to be desired. I had a Rio that I used for lack of other options.
For the iPad, it's not so simple. Most people don't have tablets, so there's no palpable need to be relieved of frustration. Instead, users are left comparing it to devices that they like: Their laptops, iPhones and iPods. The solution, of course, is to get one in people's hands. I can say first-hand that once you use it, you get it.
This time, that will take longer than Apple is used to.