Should capacitors be placed on the USB D+/D- lines? What about pull-up resistors? How can the D+/D- be protected from noise?
Unless stated otherwise in a datasheet or errata, capacitors and resistors (even ferrite beads not intended for use with USB devices) should not be added between the C8051 USB MCU and the USB connector. These external components can potentially cause the USB device to fail USB-IF compliance testing and inhibit proper communication over the USB bus. The D+ and D- lines should be connected directly from the MCU to the USB connector.
Silicon Labs recommends using protection diodes intended for use with USB devices for ESD protection. One example of such diodes is the Littlefuse SP0503BAHT recommended in the USB MCU and CP21xx datasheets.