The SAME thing happened to me, and I did some searching and found that by pressing control and shift right after you plug it in hold it down until the ipod is recognized and VOILAA. It doesn’t sync :]

Don’t auto sync because ipod can only sync with one computer itunes or the ipod files will be erased. itunes blocks this function since itunes is the tool with one way to transfer files from itune to ipod. you must utilize other tools to achieve that. There’re couple of tools for transferring the files between ipod and computer (for backup)depends on your demands, e.g transfer only songs/videos, transfer photos etc. You can have a look of below review site, hope it useful.

Set your iPod to manually manage music;
But the good way is to transfer all existing songs/videos from your iPod back to your new computer and iTunes. Simply use this handy transfer program called Tansee iPod Transferhttp://www.gutensoft.com/Utilities/Tanse…
Many of my friends are using it for its easy and safe transfer.
With just a few simple clicks, it will help you copy all videos/songs from iPod back to your selected folder on your new computer, and then you can easily add those files back to iTunes.
More importantly, this won’t overwrite any original files on your iPod. And all song titles are preserved.
In this case, all songs/videos (both new and old) are on your iTunes library that you can syn with whenever you wish to without deleting any files.
It even has step-by-step guide on the whole process:http://gutensoft.com/Guide/How-to-transf…
Best luck!