This is in the nature of a warning, I guess. I bought an Arduino 1280 Mega clone on eBay from a seller/manufacturer in China. It seemed to work pretty well, but with some problems. For example, 2x16 LCD displays would only work with the LCD4Bit_mod library, and not with the LiquidCrystal one (even when modifying the sketches for RS, Enable, and data pins). And the serial monitor examples didn't work at all.
Well, then. I got another Arduino clone via eBay from China (when will I ever learn?). This one is a duemilanove clone. And this one seems to be 100% compatible, as far as I can tell so far. Everything that didn't work with the other clone works fine with this one.
The moral of the story is to buy official Arduinos when possible, or otherwise be aware that there may be hardware/software compatibility problems. Hey, don't know if anyone has posted about this before, but it's an issue people here should be aware of.