Yes, but for that to work you need a portrait on-screen keyboard at the OS level. That is the biggest concern for me, that it does not have all the needed support in the OS to make portrait support in applications easy. Just being able to ask the OS if you are in portrait or landscape mode is NOT enough, you need common portrait UI elements so that you keep UI consistency in portrait mode between applications.

Personally I know I would use portrait to quickly go into conversations, write quick one-liner replies to SMS/IM messages.

How often do people ask you a yes/no question and being able to quickly tap "yes" in portrait mode, when you only have one hand free, would be useful. Its far more practical using portrait mode on the bus as your elbows stick out more using both hands and the seats are always too narrow. If someone is sitting next to you, landscape is a real chore.

Also as pointed out previously, many websites despite being written in landscape on the PC, they actually are portrait because its harder to read landscape pages, you get information overload if too much is on a single line.

Once you accept that a few core applications need portrait support, it makes sense to then think about the home screen. At the very least, it should have a portrait optimised quicklaunch or something, perhaps that only lets you access portrait mode applications. I personally like the idea of having a separate desktop for portrait mode which you customise for what you will most likely be doing in portrait mode. I do not think it will be any more confusing to the end-user than being stuck in landscape is now.