Hmm...could this possibly be a 3GS only feature?
There are a bunch of options.
Voiceover - tells you what you push
Zoom - enables zoom in any application by double tapping with 3 fingers
White on black - inverts colors
Mono Audio - self-explanatory
Speak Auto-Text - Speaks auto-corrections when typing
the 3G does have 4 'notches' in the headphone jack, as does the 1st gen iphone. this is what enables the in-line remote on the iphone and iphone 3gAnd before anyone complains about it being a 'software only thing' the reason it works with the 3G S is because the 3G S has a fourth notch in the headphone jack, where the 3G does not. This is what enables voiceover, and the volume switches on the new headphones (And my in ear Apple ones.)