These "tricks" are scattered around the web, but I have not found a compact, to-the-point post with all the shortcuts that we can use on the on-screen (stylus) keyboard and with the hardware keyboard, so here it is:
N800/N810 on-screen stylus keyboard gestures
- Space: Tap and move right
- Enter: Tap and move down
- Backspace: Tap and move left (keep the stylus on the touchscreen and move to the right, then left again, etc.. for multiple backspace keypresses)
- Shift inversion: Tap the character you want shifted and move up (also works for the number block and its special characters)
N810 hardware keyboard shift+Fn alternatives
- Shift single character: Press shift, release shift, then press character
- Single "blue" character: Press Fn, release Fn, then press character
- Shift lock: Press Shift twice (press shift once to leave the shift lock mode)
- Fn lock/"Number mode": Press Fn twice (press Fn once to leave the Fn lock mode)
The Single "blue" character shortcut is helpful to enter special characters and numbers on the left side of the hardware keyboard with your left thumb only, as there is no Fn key on the right side of the keyboard.
Update: Timeless pointed out that these are well documented in the OS2008 "Help" application under "Text input methods" and then "Virtual keyboards" and "Integrated keyboard". The latter even includes another hint for entering accented letters and umlauts: Hold down Chr and press the letter key repeated until the desired accented letter appears. Even if you normally don't read the online manual, these two pages are worth reading.