I want to post entire boot message to help you to see better my problem.
Maybe I posted not sufficient elements:

$ src/linux-2.6.18/linux ubd0=rootfs.debian.etch ubd1=swapfs.debian eth0=tuntap,,, noprocmm mem=512M
Checking that ptrace can change system call numbers...OK
Checking syscall emulation patch for ptrace...missing
Checking for tmpfs mount on /dev/shm...OK
Checking PROT_EXEC mmap in /dev/shm/...OK
Checking for the skas3 patch in the host:
  - /proc/mm...found but disabled on command line
  - PTRACE_FAULTINFO...not found
  - PTRACE_LDT...found
UML running in SKAS0 mode
Checking that ptrace can change system call numbers...OK
Checking syscall emulation patch for ptrace...missing
Linux version 2.6.18-skas3-v9-pre9-fremap (root@Gentoo) (gcc version 4.1.1 (Gentoo 4.1.1)) #1 Wed Oct 11 21:10:51 CEST 2006
Built 1 zonelists.  Total pages: 131072
Kernel command line: ubd0=rootfs.debian.etch ubd1=swapfs.debian eth0=tuntap,,, noprocmm mem=512M root=98:0
PID hash table entries: 4096 (order: 12, 32768 bytes)
Dentry cache hash table entries: 65536 (order: 7, 524288 bytes)
Inode-cache hash table entries: 32768 (order: 6, 262144 bytes)
Memory: 506112k available
Mount-cache hash table entries: 256
Checking that host ptys support output SIGIO...Yes
Checking that host ptys support SIGIO on close...No, enabling workaround
Using 2.6 host AIO
NET: Registered protocol family 16
NET: Registered protocol family 2
IP route cache hash table entries: 4096 (order: 3, 32768 bytes)
TCP established hash table entries: 16384 (order: 5, 131072 bytes)
TCP bind hash table entries: 8192 (order: 4, 65536 bytes)
TCP: Hash tables configured (established 16384 bind 8192)
TCP reno registered
Host TLS support detected
Checking host MADV_REMOVE support...MADV_REMOVE failed, err = -22
Can't release memory to the host - memory hotplug won't be supported  <---------------   O_O  !!!! ??????
mconsole (version 2) initialized on /home/flavio/.uml/iftCTz/mconsole
Netdevice 0 : TUN/TAP backend - IP =
VFS: Disk quotas dquot_6.5.1
Dquot-cache hash table entries: 512 (order 0, 4096 bytes)
io scheduler noop registered
io scheduler anticipatory registered (default)
io scheduler deadline registered
io scheduler cfq registered
TCP bic registered
NET: Registered protocol family 1
NET: Registered protocol family 17
Initialized stdio console driver
Console initialized on /dev/tty0
Initializing software serial port version 1
 ubda: unknown partition table
 ubdb: unknown partition table
kjournald starting.  Commit interval 5 seconds
EXT3-fs: mounted filesystem with ordered data mode.
VFS: Mounted root (ext3 filesystem) readonly.
Usage: init {-e VAR[=VAL] | [-t SECONDS] {0|1|2|3|4|5|6|S|s|Q|q|A|a|B|b|C|c|U|u}}  <----- What does it want???
Kernel panic - not syncing: Attempted to kill init!

Modules linked in:
Pid: 1, comm: init Not tainted 2.6.18-skas3-v9-pre9-fremap  <--------   I don't know what does it mean.. I never saw it.
RIP: 0033:[<00000000404031ef>]
RSP: 0000007f7f9fdc28  EFLAGS: 00000246
RAX: ffffffffffffffda RBX: 00000000405a3af8 RCX: ffffffffffffffff
RDX: 0000000000000001 RSI: 000000000000003c RDI: 0000000000000001
RBP: 0000000000000001 R08: 00000000000000e7 R09: ffffffffffffffc0
R10: 0000000000000000 R11: 0000000000000246 R12: 00000000405a3af8
R13: 0000000000000000 R14: 0000007f7f9ff078 R15: 0000007f7fa00fc4
Call Trace:
6176bb88:  [<60013cc7>] panic_exit+0x2f/0x49
6176bba8:  [<60038a63>] notifier_call_chain+0x29/0x3e
6176bbd8:  [<60038b08>] atomic_notifier_call_chain+0x9/0xb
6176bbe8:  [<6002b69a>] panic+0xe6/0x178
6176bc58:  [<600fbd21>] tty_write+0x1fa/0x215
6176bc78:  [<601009b0>] write_chan+0x0/0x20d
6176bc88:  [<60023d22>] wait_stub_done+0x6f/0x124
6176bcd8:  [<6002df7d>] do_exit+0x7b/0x3fe
6176bd18:  [<6002e3eb>] sys_exit_group+0x0/0x14
6176bd28:  [<60067995>] sys_write+0x47/0x70
6176bd58:  [<6002e3fd>] sys_exit_group+0x12/0x14
6176bd68:  [<600146ad>] handle_syscall+0x65/0x80
6176bd98:  [<60023fc6>] userspace+0x1ef/0x237
6176bdf0:  [<6000f1e2>] init+0x0/0x132
6176bdf8:  [<600142d5>] new_thread_handler+0xb8/0xca

2006/10/10, Flavio < fbcyborg@gmail.com>:

Is there really no space between the "eth0=tuntap,,," and
"noprocmm"?  If so, that would cetainly break something.

No there isn't. I don't know why copy & paste cause this effect on the mail message.
I also added mem=512M parameter, but I'm getting the same result.
Maybe there's some problem with new kernels only.