The new iPhone announced this week does sound very nice and all, and the new gyroscope might have some fun uses, but I think it's still missing one crucial sensor: a thermometer.
Why is this important? Because temperature is something that has a big impact on our lives, and that people talk about a lot, and yet most of the time we don't have access to any accurate measure of it. Consider the following dialogue, which seems to take place fairly frequently:
Person A: "I'm a bit hot. Is anyone else hot? Shall I open the window?"
Person B: "Oh, I'm okay. It doesn't feel hot at all to me."
Person A: "Really? I'm roasting. It's definitely warmed up."
Person B: <shrugs> "I'm okay..."
(The situation can also be reversed, where one person feels colder than everyone else).
These disagreements prove that the sensation of temperature is subjective, and also that we're often unaware of temperature changes until it starts to make us feel uncomfortable. In both cases, being able to glance at the actual temperature would immediately act as a social talking point. Everyone who has a digital thermometer in their car (of the outside temperature) always seems to pay quite close attention to it.
There could be other uses of a temperature sensor in your mobile too. For a start, it would require such low power, that an application would be able to take regular readings in the background, and plot charts of your experience of temperature over the course of weeks and years. So, for example, you could have a personalised notification of "this is the hottest/coldest day of the year so far".
It'd also be a way of resolving arguments over whether this winter/summer is colder/hotter than last year (given that people have really bad memories of this).
So there's my tip to Apple. It wouldn't even cost much - the only thing needed is a cheap, tiny thermistor - and it would be another point of differentiation.
In the meantime, here's the chunky alternative that I use (which has been clumsily styled as a classic iPod):