From what I've read, we know that the Tab only draws .5A power from a USB socket, but that it draws ~2A from the wall, and that it knows the difference by testing for a short on the TX/RX USB data wires. (Correct me if I'm wrong on the details, but that's the general impression I've gotten.) We also know that the iPhone/iPad/iPod uses a pair of resistors instead (per the USB spec) to determine the current-delivery capability of the source. (I'm no EE, so I'm a little fuzzy on how that works from a technical perspective.)
So to set the stage: I had brought the USB cable for my Tab with me, but had neglected to grab the USB->outlet adapter for the mains. Similarly, a friend who was on vacation with us had forgotten the charging cable for his iPhone. Thinking I was killing two birds with one stone, I bought an aftermarket USB charging cable for the iPhone which had a separate USB->outlet adapter. His iPhone charged swimmingly, and when I charged my Tab overnight (screen off, starting at ~89% charge) using my cable and the aftermarket adapter, it worked flawlessly. No problem, great solution, or so it seemed...
...until I got home. Suddenly, my Tab wouldn't do more than a trickle charge - the same charge it gets when connected to the USB port on my laptop. (I could tell this was the case because of the small red 'x' next to the charging symbol in the battery icon - which I hadn't noticed while on vacation.) A 2.5-hour car trip using Navigator (screen on the whole time), even with the car charger connected, killed the battery completely. No matter what I did - shutting down apps, rebooting the device, resetting every option I could find - I could not get a fast charge to happen. Even leaving the device plugged in overnight resulted in *less* charge than what I started with if I didn't turn the screen off. I called T-mobile, talked to Samsung tech support twice... and all they could offer was to have me send the Tab back for replacement.
I was at a loss. I nearly did a hard reset in frustration, thinking I had screwed something up royally.
Then I saw the resistors in the pictures in this post:
and something clicked. I had been assuming that the *cables* were the difference between the iPhone and Tab chargers... but there isn't enough room for resistors in the cables! (Well technically, there is, but regardless...) The resistors must live somewhere else... and, I reasoned, that somewhere must be inside the USB->outlet adapter. So I pulled out the 'proper' Samsung adapter, plugged in the Tab, and voilą... a perfect fast charge.
So this is why the Tab won't charge with an iPod (or Sony-Ericsson, as it turns out) mains USB adapter OR USB-port car charger: the circuitry to let the device know it's capable of a full 2A delivery doesn't live in the Samsung cables; it's only present in the adapter. If that's not there, the device thinks it's connected via USB (and will, in fact, say "USB Connected" every time the charger is plugged in) and won't charge more than a trickle.
So that's my story, and hopefully it can save someone else some pain and frustration. Thanks for giving me the floor.