On photo trips, where I do not have a computer with me, I would like to twitter or email some JPEG images I have just taken with my camera. The photos sit on a SD card, and should go to the iPhone for resizing / cropping before sending them off. I am willing to spend some money for extra hardware to do this (adapters, wireless stuff?)
I know there is a connection kit for the iPad, but I could not find a solution for a non-jailbroken iPhone.
This can be accomplished pretty easily just using an Eye-Fi SD card (a standard SD card with a built in wi-fi transmitter) and the corresponding Eye-Fi app on your iPhone. The app connects to the card via an ad-hoc network allowing the photos to be transferred.
Eye-Fi cards start around €20 and the app is free on the App Store.
More information on the process can be found here.
Eye.fi makes SD cards that will transfer the photos to their app on your iPhone or iPad directly. It's called "direct mode".
If you have an iPad already, you can always share your internet connection from your iPhone via Personal Hotspot, then when you transfer the pictures with the camera connection kit, you can email/tweet them as if you had a regular wi-fi connection.
Other than that, recommend the Eye.fi solution proposed by others.
