The CDC data interface specifies a USB poll interval of 0. See here:

Code:
#ifdef CDC_DATA_INTERFACE
        // interface descriptor, USB spec 9.6.5, page 267-269, Table 9-12
        9,                                      // bLength
        4,                                      // bDescriptorType
        CDC_STATUS_INTERFACE,			// bInterfaceNumber
  ...
        CDC_DATA_INTERFACE,                     // bInterfaceNumber
        0,                                      // bAlternateSetting
        2,                                      // bNumEndpoints
        0x0A,                                   // bInterfaceClass
        0x00,                                   // bInterfaceSubClass
        0x00,                                   // bInterfaceProtocol
        0,                                      // iInterface
        // endpoint descriptor, USB spec 9.6.6, page 269-271, Table 9-13
        7,                                      // bLength
        5,                                      // bDescriptorType
        CDC_RX_ENDPOINT,                        // bEndpointAddress
        0x02,                                   // bmAttributes (0x02=bulk)
        CDC_RX_SIZE, 0,                         // wMaxPacketSize
        0,                                      // bInterval
        // endpoint descriptor, USB spec 9.6.6, page 269-271, Table 9-13
        7,                                      // bLength
        5,                                      // bDescriptorType
        CDC_TX_ENDPOINT | 0x80,                 // bEndpointAddress
        0x02,                                   // bmAttributes (0x02=bulk)
        CDC_TX_SIZE, 0,                         // wMaxPacketSize
        0,                                      // bInterval
#endif // CDC_DATA_INTERFACE
What effect does this have? i.e. what poll interval will be chosen?
And: is there a reason why this is not configurable as the intervals for the other devices (like KEYBOARD_INTERVAL)?