That sounds odd, I find it hard to believe that something got so hot from a USB charge.
The thread I saw was pretty simple:
1. Passthrough left plugged in and it never shut off. It was an automatic, IIRC.
2. Owner walked away, came back and the atty (since it was being fed constant power) had become red hot, burning what it was sitting on.
It's hardly common, but it highlights again that these are consumables. You may eventually have batteries that refuse to shut off as well, and reports of that are far more widespread. None of the commodity stuff we get for $10 out of China is going to have a strict QA process. Be careful with your stuff.
Now, that being said, I have two 510 passthroughs (bought from Eastmall, though they appear on other sites) and I've never had any issues with them. The paranoia isn't really justified, though I'm sure we'll hear reports of a dodgy 510 PT at some point. If you're really concerned and want a quality bit of engineering go for a Puresmoker USBPass.
Of course, that only applies if you live in the US. Stupid lack of international sales... :/