Strange - my Inspiron 6000 has them documented in the Owner's Manual.

Here is what my keyboard has (I suspect that it will be similar to yours)

Fn+Esc (Standby) puts laptop into standby- you may be able to change this in Windows Power Management

Fn+F1 (Hibernate) puts laptop into hibernation

Fn+F2 (antenna symbol) toggles wireless and Bluetooth cards off & on

Fn+F3 (battery symbol) displays Dell QuickSet battery meter

Fn+F8 (CRT/LCD) used with an external monitor to determine which screen is used - internal, external or both (mirrored)

Fn+F10 (CD halfway out of drive) eject CD tray

Fn+PrntScrn (SysRq) generate the SysRq keystroke

Fn+NumLk (ScrollLk) generates the Scroll Lock keystroke

Fn+Pause (Break) generates the Break keystroke

Fn+PageUp (Speaker with 3 lines) increase volume

Fn+PageDn (Speaker with 1 line) decrease volume

Fn+End (Speaker with X over it) mute speakers

Fn+UpArrow (Sun up) increase backlight brightness

Fn+DownArrow (Sun down) decrease backlight brightness

Fn+ 7 8 9 0 u i o p j k l ; m . / is the numeric keypad (may need NumLock enabled otherwise you get Home Up PgUp etc)

My Owner's manual also mentions Fn+F4 and Fn+F5 as toggle Num Lock and Scroll Lock but they are not marked on my keyboard and seem to do nothing.