Disclaimer: all these are my personal opinions with the N900. Maybe other N900 users want to to add their impressions to the mix. Also note than it is possible to handle the device in landscape mode and do some basic operations as well. I mean, landscape doesn't mean automatically two hands.

Browser, what is the strong point for portrait mode? As long as you are ok with 480px width and you don't need to input much text... But I agree is an interesting case and worth submitting a proper proposal at http://maemo.org/community/brainstorm

SMS texting implies a portrait keyboard that we don't have nor is planned as for today in Fremantle. Are you SO in a hurry that you can't stop few secs to text with a convenient qwerty you have paid for? Typing while running is dangerous anyway, no matter in which mode.

shopping list --> feature request to whoever develops the app.

calendar (for browsing), it's indeed in landscape mode but it ism possible to browser with one hand (at least for me, I have relatively big hands). Fitting a week of 7 days in a single row in portrait mode is not that simple and probably would require serious work on the UI.

calculator, doesn't look that complicated to do but is it such an important use case? Note that real calculators tend to be used with two hands, even the ones that are small with portrait shape. In any case you can always ask this guy to port his clone (or other guys porting other clones).