If I sell my iPhone, how will the warranty be affected? I recently dropped my iPhone 4 in the bath. Whoops...

Knowing Apple's stance on water damage, I simply upgraded (I was due an upgrade as part of my contract) and got a new phone.

My house-mate then informed me that I could go to Apple and purchase a new phone for £119. I did just that, and the kind lady in the Apple store let me know that the new phone only comes with a 90 day warranty.

As such, I have two Apple phones, and I'd like to sell my old (but brand new..) iPhone 4.

My question is: if someone purchases my phone on say eBay, will they too receive the 90 day warranty, or does it become invalid when I sell the phone?
The original warranty usually the standard 1 year + any AppleCare stays with the iPhone. And any remaining warranty transfers to the new owners.
Apple Care FAQs

Is the AppleCare Protection Plan transferable?
Yes. If you choose to sell or give away your Apple product, you can also transfer the ownership of the AppleCare Protection Plan. Please see the AppleCare Protection Plan Terms and Conditions for complete details.