Is there a use for my old iPhone?
You could:  


Sell it: Gazelle is a good place to sell old gadgets.  
Use it as a regular iPod Touch (I gave my old iPhone to my teen to watch iTunes U videos for school).  
Jailbreak it.   
Test Custom Apps  
Turn it into a neat personal project (remote control cars/planes, house automation).  Blog about it too.   
Donate it   
Replace your car stereo (this one is fun).  
Make it your portable DVR; load it with your favorite shows and watch them when you have time (I know you can do this with you new iPhone, but it'll save on battery life if you use a second device).   
High tech alarm clock (I know, that one's silly).


Sorry if I repeated any suggestions.  I was doing a quick brainstorm.  It's fun finding new ways to use old gadgets.  Good luck and let us know what you do.