The N900 is a phone with a good browser and some limited computing capabilities but with a teeny screen with limited graphics, a teeny thumbboard and no cursor capabilities on its own.

A netbook has no phone (other than Skype or similar software), a better browser, and better computing capabilities with a bigger screen and keyboard and a cursor.

There are also UMPCs (including my OQO) and MIDs that fall somewhere in between.

It all boils down to what you want to do and where you want to do it. Realistically, if you are looking for something to power a large screen with a keyboard and mouse you'd do better to invest in a netbook than in a bluetooth keyboard and mouse. The low end ones aren't that much more than a good keyboard/mouse set and they will beat the N900 in every way except making phone calls.