Sainsburies sell a charger that supplies 1000mA and will do fine. They also sell one that supplies 500mA (IIRC) and wont. Whichever one you get, there's change from £5.
I believe that the Micro USB on the Pi is actually for power only with the data pins being either not connected or ommited all together. The other USB ports are not equipped to handle input power.
The input USB (Micro B) to the Raspberry PI PCB has a resetable fuse which trips at 1.5A nominal - dead safe at 700mA. These are really to catch dead shorts
Each USB is similarly fused at 280mA trip 140mA nominal. (Raychem MINISMDC014F-2 if you want to look at the data). We looked at the more sophisticated USB port controllers and decided that they just cost to much for the project to handle.
So if you have a beefy enough supply you can suck 160-200mA out of each USB without any trouble 250mA at a push, the full 500mA (*2) would over stress the PSU that most people will use so we capped it back.
If you need more, a powered hub is the best bet. You can route that back the Micro USB B to power the RPi. As Draxy518 points out there are no data pins connected so it is OK. To get the data connection you have to wire one of the Standard USBs to the hub anyway.
On the Model A you only have one USB so a hub is needed to to get mouse and keyboard.
