Thanks for your explanation. I've also found more information here about the need to pull the signal voltage to 3.3V:

http://www.beyondlogic.org/usbnutshell/usb2.shtml

Just for my understanding: Am I right that the capacitors serve to filter the supply voltage? I was also wondering why two capacitors are needed. I guess this has something to do with filtering different frequencies (e.g., the large cap filters out low frequencies, and the small one the high frequencies)?