Comment 2 for bug 900384

This also affects me. I have Ubuntu 10.04 with installed kernels from the repo from 2.6.32-21-generic
 to 3.0.0-15-generic. And looks like developes changed kernel config in 3.x branch to turn off usbip drivers compilation.

2.6.38 usbip Makefile is
  1 obj-$(CONFIG_USB_IP_COMMON) += usbip_common_mod.o
  2 usbip_common_mod-y := usbip_common.o usbip_event.o
  3
  4 obj-$(CONFIG_USB_IP_VHCI_HCD) += vhci-hcd.o
  5 vhci-hcd-y := vhci_sysfs.o vhci_tx.o vhci_rx.o vhci_hcd.o
  6
  7 obj-$(CONFIG_USB_IP_HOST) += usbip.o
  8 usbip-y := stub_dev.o stub_main.o stub_rx.o stub_tx.o
  9
 10 ccflags-$(CONFIG_USB_IP_DEBUG_ENABLE) := -DDEBUG
via http://lxr.free-electrons.com/source/drivers/staging/usbip/Makefile?v=2.6.38

and 2.6.38-13-generic config has necessary values set:
$ egrep USB_IP /boot/config-2.6.38-13-generic
CONFIG_USB_IPHETH=m
CONFIG_USB_IP_COMMON=m
CONFIG_USB_IP_VHCI_HCD=m
CONFIG_USB_IP_HOST=m
# CONFIG_USB_IP_DEBUG_ENABLE is not set

But if we look at 3.0 (and later) usbip Makefile:
  1 ccflags-$(CONFIG_USBIP_DEBUG) := -DDEBUG
  2
  3 obj-$(CONFIG_USBIP_CORE) += usbip-core.o
  4 usbip-core-y := usbip_common.o usbip_event.o
  5
  6 obj-$(CONFIG_USBIP_VHCI_HCD) += vhci-hcd.o
  7 vhci-hcd-y := vhci_sysfs.o vhci_tx.o vhci_rx.o vhci_hcd.o
  8
  9 obj-$(CONFIG_USBIP_HOST) += usbip-host.o
 10 usbip-host-y := stub_dev.o stub_main.o stub_rx.o stub_tx.o
via http://lxr.free-electrons.com/source/drivers/staging/usbip/Makefile?v=3.0

and 3.0.0.13-generic config:
$ egrep USBIP /boot/config-3.0.0-13-generic
# CONFIG_USBIP_CORE is not set

there are no necessary values set and usbip is not build by default for 3.x generic kernels.