First off, I'll take a keyboard where they key mapping is done with xkb over a keyboard where it's not, almost universally. That, more so than any particular key availability or layout, matters to me. Because of the beautiful simplicity that if you have that, you can set what keys you do have to be much more suitable for you.

Want a tab key? Easy, find some unused key combination (or one used for something you don't care for), and use it. For me, that's Fn+Enter on the N900.

Even on the N900 with its 3-row, rather limited keyboard, I have just about any key I want, the limitation being more about how many key layouts I can memorize and comfortably use. I have Fn+Shift (in that order. Shift+Fn is just another key level in one layout) mapped to cycle through my key mappings. I have the latin/english characters and all the commonly used specials in that layout, cyrillic/russian characters and many other special characters on a second one, and have been in the process of putting together a third layout for all the special characters that typically come up in mathematics or science. I can certainly type more things in at will than is available (near as I can tell, if the only keys available are the ones printed on the buttons) on the N950 keyboard's default key mapping.

Secondly, if the above is satisfied I'm perfectly okay with 3-row keyboards. 4-row and 5 row would also be acceptable to me, thought I would conjecture this would make touch-thumb-typing harder, and 5 seems like pushing it, but if overall it's comfortable to use, great. Bigger keyboards typically mean they start to expand, which means you need bigger and bigger hands to comfortably reach all the keys with your thumbs. This has more to do with the size of the device than the key placement, though. The black-berry Z10, for example, from my fiddling with one, is sometimes noticeably uncomfortable for me because it's size means I actually end up having to 'reach' with my thumbs. I can tolerate it because it doesn't come up often, but I would absolutely hate being thus inconvenienced with a keyboard. (Admittedly, usually on a keyboard you don't have to reach across more than half the span thereof with a given thumb, though depending on exactly what key combinations you've mapped to what, and whether the modifier keys like shift and Fn 'latch' or need to be held to have their effects. On the N900 for example, shift and fn latch, but some programs, like emacs, seem to disregard this latching and then you have to held them pressed down to get the desired characters).

So the point is, I really don't have two much of an ideal layout. It should have arrow keys, and ideally the full set of modifier keys (Ctrl, Shift, Fn, Alt, Meta), although the minimal N900 set (Ctrl, Shift, Fn) has worked well enough for me most of the time. Give me xkb on the software end, and I can take it from there, thanks.