the 2.6.1 UML snapshots do not seem to boot FS images that worked
before:

 NET: Registered protocol family 1
 NET: Registered protocol family 17
 Initializing software serial port version 1
  /dev/ubd/disc0: unknown partition table
 Initializing stdio console driver
 VFS: Cannot open root device "ubd0" or unknown-block(0,0)
 Please append a correct "root=" boot option
 Kernel panic: VFS: Unable to mount root fs on unknown-block(0,0)

this is with 2.6.1 + uml-patch-2.6.1-1. I've attached the .config. 
Despite the kernel's message, the config has UBD support included:

 CONFIG_BLK_DEV_UBD=y

the ubd symbols are in the kernel:

 $ grep ubd System.map  | grep open
 00017d84 t ubd_open_dev
 00018614 t ubd_open
 00019725 T open_ubd_file

so it ought to work, but doesnt.

same FS image works fine with the older 2.6.0 uml combo patches. Host
kernel version is irrelevant to this problem - tried a number of 2.4 and
2.6 kernels.

	Ingo