Well, with that tool, you can only pull out data, but not put it back in later. Whatever app data you pull out would become useless to you.
The way I was talking about, Jailbreaking, would require jailbreaking the device, and then using SSH to connect to the device to gain access to the file system. You could then back up the app's save data from the installion folder (if it does exist there, I have not tried this myself, so it may be stored elsewhere) to your computer. Once you got a different device, you'd have to install Myst, jailbreak the device again, and then put the save data back where it was.
That way is a lot more work than simply using the backup restore feature of iTunes, and the jailbreak is semi-permanent. (You need to do a restore of the device to undo it.) If you don't need/want a jailbreak the method is undesirable. I say that I don't consider it low level hackery because I think that jailbreaking is fairly large hacking, and then you have to install further software for accessing the filesystem, like the SSH software.
You might be able to use the backup slurper to get the save data out, and then restore it using the jailbreak, but it's not something I've looked into, and it's still overkill next to using the iTunes backup and restore.
Edit: Oh, and I don't believe there's a jailbreak yet for the iPhone 3G S anyway, due to to it's newness.
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