There is no limitation on USB currrent in rev 2 board other than:
- how much traces would be able to handle. 2.5A should be safe.
- how much current you can get from input. There is a 1.1A polyfuse (with 700mA "hold current") on microUSB port. Since RPi itself is using somewhere between 300mA to 550mA, not much is left for USB devices. If you are using other way to power your RPi up, this limitation does not apply.
Keep in mind, however, that hotpluging any USB device that takes much current may reboot your RaspberryPi because of voltage drop. So its suggested not to use high current USB devices or plug them before booting.
And yes, you can now (almost) safely power your RPi from USB if your device back feeds power. Keep in mind, however, that USB specification says that devices should not do that (but some USB hubs and probably other devices do not have protection about back feeding power despite what specification says). Since there is no current limit, you should watch out for devices that could give more than 2.5A of power at faulty conditions as this may burn traces on your PCB.