Tim Bray, in his increasingly simple blog, boldly states that tablets are meant to be held in portrait orientation:

Tall and Narrow
http://www.tbray.org/ongoing/When/20.../Portrait-Mode

There's quite a lively debate on this.

What do you think is more natural? Portrait or landscape?

The reason I ask, is because I remember when Nokia was refusing portrait orientation on the N900. This annoyed a few folk at the time, but has important usability/design implications. Now, a tablet is larger than a phone, but it is interesting to consider.