Most of our users connect the AVCC to the VCC and the AGND to the GND. The requirement depends on how much noise is in your digital system. If you are bus powered and have a 3.3 regulator without much local noise on the power you can connect the two. If you are self-powered it still depends on how noisy the power is. The AVCC supplies the power to the bus drivers and to the clock. If power was noisy it would show up as jitter in the USB data. Too much would cause signal quality problems. If you have little or no noise, connecting the digital and analog points together is not a problem.
They are separated only to allow isolations in systems, which have high digital noise. All of the designs we have seen so far have used a single power source and connect AVcc to Vcc and AGND to GND.
Please go through application note EZ-USB FX2(TM) PCB Design Recommendations - AN1196 to find a section on power supply decoupling.