Currently, when navigating single-line text fields within Maemo 5, using the UP and DOWN arrow keys on the hardware keyboard move the cursor to the previous (UP) and next (DOWN) text fields, if more than one text field is present (e.g, modifying settings in Modest, or adding information to Contacts, etc.). If only one text field exists within a View, the UP/DOWN arrow keys do nothing.
In Maemo Browser, the UP/DOWN arrow keys do nothing, and navigating back and forth between various single-line text fields require the user to use CTRL+LEFT (previous text field) and CTRL+RIGHT (next text field).
In either case (whether in the Maemo 5 UI or on a web page in Maemo Browser), using SHIFT+ARROW does nothing.
NOTE: My personal preferences are Solutions 2-4, as they would most closely resemble (not exactly, but close enough) the functionality of a desktop/laptop computer. Normally, the TAB key would solve some of this, but since one isn't available on the N900's keyboard, other means are necessary.