I've had phones with landscape and portrait mode keyboards, and the latter were invariably better. Maybe it's just that Palm and RIM were better at making keyboards, but I'm not interested in a landscape keyboard at this point
I would put an offensively large bet on BlackBerry putting absurd amounts of effort into their keyboards, instead of landscape being inherently worse. Based on everything I've ever seen, heard, and read, they cared deeply about hardware design. "Losing the Signal", regarding the rise and fall on BlackBerry, made it clear they tested for things like drops extensively, and (IIRC) went as far as literally throwing at least one device around to ensure it would survive.
And then they were pushed to push the Storm out the door, which pushed many people away. Things only really went downhill from there, sadly. That said, my KeyOne is still my favourite phone ever, and I'll carry on with it as long as I can purchase replacement components :)
On further reflection, I think that it's just not that much worse to two-thumb type on a narrow keyboard, and a landscape isn't big enough to five finger type, but to big to single-thumb type.
