an embarrasingly simple question here

You have to get the music files off your iPhone to the new computer. iTunes won't do this for you (to help deter music sharing).

Google 'iRip'.

 

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the beauty of iTunes...

You can't!

 

Don't be embarrassed, you just discovered the stupidest thing about iTunes!

You'll have to re-sync it (if you have all of your 3G's music backed up) and if not, you'll have to find a program compatible with your model iPhone that pulls the music off.

 

That's one downside of using itunes. Lets you only sync with one itunes library and once you try to sync with another it tries to erase all the old info

 

When my hard drive crashed on my Mac over the summer I used this to extract music off my iPad that hadn't made it onto the last Time Machine back-up: http://www.macupdate.com/app/mac/35334/detune

 

It's also the downside of not backing up.

 

iTunes is my least favorite part of anything Apple, I've lost so much music over the years. Frustrating to figure it out but I'm sure Apple likes that as it makes them more money when you have to buy your music again. Grrr.....