Definitely, but from my experience with the N97 (which has very similar form factor and functionality) there are many many many situations where you would want to use the N900 in vertical mode too.

For example if I'm walking with a bag I simply cannot hold the device in two hands, but I may still want to access some of its functions (for example changing tracks on the music player, checking my e-mail, changing radio stations etc).

And if you want to write with one hand (which many people do on messages of various kinds) you'd need a vertical mode with a virtual keypad.

One of the main reasons the Nokia 5800 has sold so well is possibly because it is a touchscreen smartphone that works just as well one-handed as it does two-handed. Its narrow screen and vertical modes for almost every application mean you can use it just like a normal mobile phone if you wanted to. It even has a virtual predictive text keypad which I find very useful for messaging, my text entry speed is extremely high on the 5800's virtual keypad because I've been using predictive text mobiles for about seven years now.