Additional: If you're out somewhere and see a great spot, but you only have your iPhone, take a photo anyway.
Then utilise the geo-tagging feature and return to take the 'real' photos with your SLR.
I bought my iPhone during the great o2 price drop of '08, for £169.
I recently sold it for £235.
So effectively, I've been paid somewhere in the region of £60 for using an iphone for a month or two, AND I now have enough cash to get myself a nice new 16Gb 3G version.
All in all, I very much feel that I've done alright out of this upgrade.
However, if it hadn't been profitable for me to do so, I wouldn't have upgraded. 3G coverage here might be great, and GPS is likely to come in useful, but neither of them are big enough to outlay the sort of cash needed. The headphone jack though...
10 Ways Professional Photographers Can Use the iPhone
What to Do With Your First Generation iPhone?