I'm usually faaaar up on the "share everything with everyone" scale, but this actually surprised even me, and I should've known this already. I don't think the behaviour is wrong, but it's definitely something to be aware of. I'm talking about the location data in EXIF tags in photos taken with an iPhone 3G/3GS.
I took the following photo at Bridgehead last weekend. Turns out if you email that photo to anyone from the phone and open it up, there's location metadata included in the file:
OSX even includes a handy little "Locate" button in the inspector. Clicking it opens Google Maps in your default browser, which locates the photo pretty damn near where I actually took it:
The potential usefulness of this is terrific and outweighs the downsides (think "view pictures posted in last 2 hours, taken near this fire"... crowdsourcing the news), but it's definitely worth keeping in mind from a privacy point of view.