iPhone migration headache

Right, I have an iphone puzzle.

I need to migrate my iphone from my old macbook to my shiny new macbook pro (with small dent after dropping it in the first week Sad )

To make things complicated my itunes library is on an external drive usually hooked up to an old imac. When i sync i hook up the macbook to the external drive.

Is there a simple way to make sure that I dont wipe any apps/contacts / text messages etc.

Should I point my macbook Pro's itunes at the same external drive? and then sync from there?

Also, what is the best way to ensure that EVERYTHING currently on the phone is still there after the transfer..?

usually i find macs to be very straightforward and user friendly but not this time!

Any help gratefully recieved....

iTunes should be easy. Just make sure your iTunes on the new Mac is pointing to that hard drive the music is on (and the songs are listed in the new library. When you sync, it shouldn't change anything. Google "transferring iTunes library to new computer" and a whole wealth of how-to sites will pop up.
When you sync your phone to the new Mac, it will automatically update the apps on the computer; you won't lose them. At least, I didn't when I went from my HP laptop to my MBP.
I don't know how you would ensure not losing your contacts/calendar stuff unless you had a MobileMe account. They are offering it for 2 months free, so I suppose you could sign up and cancel later on? My contacts and calendar are synced via MS Exchange through work so I'm not positive on that. Do you have a GMail account? If so, you can sync to that (just make sure your contacts are up-to-date first).
Hope this helps.