OK, riddle me this.

Let's say you have an iMac, a MBP and an iPhone.

Your iMac is your main PC, so you usually sync your iPhone with the iMac. However, when you're out and about, or in foreign lands, you have your MBP & you perhaps want to transfer things from it, such as purchases from your iTunes account made on the MBP, onto the iPhone.

How do you accomplish this without iTunes deleting everything on the iPhone?

You would need to have a complete copy of your library from the iMac on the MBP, I think. At least doing this when swapping between my Mac Pro and iMac seemed to keep all the data on the iPhone.

But that would require you to run something like rsync before you left the house each day...Which is a pain.

Isn't this sort of thing sorted out with iCloud?

Yes, but if you've just purchased a 2GB film on your MBP via a 3G dongle, you'll be downloading another 2GB (presumably) to your iPhone...

It also won't work if it's a non-Apple program that doesn't use iCloud but does sync through iTunes...

deleted

Though if you have access to Wifi, you could use Dropbox.

That just seems silly. You'd think that since you can only have your iTunes account activated on 5 Macs at any one time, that you'd be able to sync your portable devices between those Macs without losing all your stuff from the iPhone in the process. It should only add what's missing, not take everything off and replace it all with whatever happens to be in your library on the new Mac.

Er.. yeah, that's what it does now. It used to be the case that if you plugged an iPod into a 'new' computer with iTunes, it would attempt to erase the ipod and copy stuff from the new computer onto it.
However now (and, frankly, for quite a while) if you plug an iphone/ipod into an 'unfamiliar' computer with iTunes that has the same Apple ID registered against it, it will give you two options - erase and overwrite or copy all the 'extra' stuff from the phone/ipod onto the computer and then do a sync afterwards.

Jon