The number one thing I'd like to see on the iPhone is to be able to turn push mail on and off according to time automatically. My venerable e66 had this - it would check my mail faithfully throughout the day between 8am and 6pm and then stop. It did wonders for the battery life and for my sleep patterns. If I inadvertantly forget to disable this when I go to bed, nagios wakes me up as it's monitoring failing servers or systems.
In fact, the ability to have auto on/off dependent on time is something that the iPhone could apply to a bunch more things too - like changes of ringtones etc. It's probably too hard to do for Apple so as my old dad says - wish in one hand and pee in the other and see which hand fills up the quickest!
The other thing I'd like is for a slightly more robust design, but hey - you can't have everything. My Motorola Backflip (MB300) is more rugged and seems to go pretty well.