Synching to Macbook rather than iMac while travelling iPhone 4

It's not easy. The iPhone will sync to only one computer at a time. If you change computers all of the content from the first computer will be deleted and replaced with content from the new computer. The best I can suggest is to copy the iTunes media folder from the iMac to the Macbook. It is located in the Music folder in Finder. Make sure the MacBook is Authorized to the same iTunes store account as the iMac. When you sync you will get a warning that all content will be deleted and replaced, but, since the content is the same on both computers this will do no harm.

Thanks for the quick response.  My iTunes music is all on an external portable drive that I will take with me.  I'm more concerned about losing all my old text messages and app data.  The reason I want to be able to synch while away is so I can vary the music I have on the phone.  If it's going to be too difficult or risky, I guess I won't bother.

You won't lose messages; that warning refers to iTunes content. You have 2 options: Set the phone to back up to iCloud, or connect the phone to the Macbook, but don't sync. Right (CMD) click on the iPhone's name in the left pane and choose "Back Up". This will copy all of your app data, settings, etc to a backup folder on the Macbook. Be sure to next go to the Info tab and set the iPhone to sync to Address Book and iCal on the Macbook. You can then safely sync.