I have my iPhone 3GS jailbroken (3.0.1) with Pwnagetool (from 3.0) and during a wrong manipulation I started jailbreaking it with Purplera1n. What I really wanted to do was to gather the signature data for my phone (using the "hidden" field on Purplera1n's website and the ID obtained from the system profiler for an iPhone in rescue mode) to secure the possibility of jailbreaking or reinstalling old software in the future.

Anyway, there I was, watching my iPhone being jailbroken by Purplera1n because I DID NOT READ THE INSTRUCTIONS CAREFULLY. So yes, listen to your parents and read the instructions before attempting potentially "dangerous" stuff. Since there was no progress indicator on the jailbreaking, I thought maybe it was not too late and that it was a good idea to turn the phone off asap. It probably IS NOT a good idea. Nonetheless, I went on with it and restart the iphone. There it didn't go well of course and kept on displaying the loading Apple logo and never going further.

I decided to try to mess up more (couldn't really mess up less at that point) with my iPhone and ran PwnageTool again over the messy iPhone and voilà, it was back to normal and I hadn't lost any data.

I hope nobody will face that situation, but if someone does, at least they know there is some chance at salvaging your iPhone via this method.

However I have to assume I was lucky on this one so you should avoid doing something stupid hoping you will be as lucky as me.