http://www.raspberrypi.org/phpBB3/viewtopic.php?f=24&t=5830
If you don't want to read all that, here's a summary from Wikipedia: Originally the on-board USB ports were designed for USB devices using one "unit load" (100 mA) of current. Devices using more than 100 mA were incompatible with the Raspberry Pi, and for them a self-powered USB hub was required. However, due to user feedback, the RPF, at the end of August 2012, decided to remove the USB polyfuses which largely caused this behaviour. However, the maximum current that can be delivered to a USB port on these modified boards is still limited by the capabilities of the power supply used, and the 1.1 A main polyfuse. Also a disadvantage of the current way the modification is done is that its no longer possible to hot-plug USB devices directly into the PI, when hotplugging is necessary it can be done in a hub. -- Aram Hăvărneanu
