I 've looked through the docs and googled the heck out of this list and the 'net, but can't find a single working example of a slip setup.  

I need to use slip because I am running the UML in a VPS (with no kernel tun/tap support, exactly the situation pointed out in the docs as the only reason to use slip)

There's a lot of commentary about how slip was the first way UML was connected to the 'net and how it has drawbacks, but unfortunately the examples present in the documentation make a lot of assumptions about the host.  I get the feeling the author genuinely believed that using slip was a bad thing, and so avoided giving a useful example as a way of discouraging its use.

Here's my config so far (all taken from the docs, but not verbatim, as it would collide with my upstream router)

#HOST ./linux-2.6.24-rc7 ubda=Fedora.cow,FedoraCore5-x86-root_fs eth0=slip,

[... loading noise...]

[root@localhost ~]# dmesg
Linux version 2.6.24-rc7-dirty (jdike@tp.user-mode-linux.org) (gcc version 4.1.2 20070925 (Red Hat 4.1.2-27)) #97 Mon Jan 7 11:18:24 EST 2008
net_namespace: 64 bytes
Using 2.6 host AIO
NET: Registered protocol family 16
NET: Registered protocol family 2
Time: itimer clocksource has been installed.
Switched to high resolution mode on CPU 0
IP route cache hash table entries: 1024 (order: 0, 4096 bytes)
TCP established hash table entries: 2048 (order: 2, 16384 bytes)
TCP bind hash table entries: 2048 (order: 1, 8192 bytes)
TCP: Hash tables configured (established 2048 bind 2048)
TCP reno registered
Checking host MADV_REMOVE support...<3>MADV_REMOVE failed, err = -38
Can't release memory to the host - memory hotplug won't be supported
mconsole (version 2) initialized on /root/.uml/wOqluE/mconsole
TCP cubic registered
NET: Registered protocol family 1
NET: Registered protocol family 17
Initialized stdio console driver
Console initialized on /dev/tty0
console [tty0] enabled
Initializing software serial port version 1
Choosing a random ethernet address for device eth0
Netdevice 0 (66:35:f6:cf:fb:ab) : SLIP backend - SLIP IP =
console [mc-1] enabled
Setting slip line discipline: Invalid argument
uml_net : waitpid process 15689 failed, errno = 10
slip_tramp failed - err = 10

[root@localhost ~]# ifconfig
lo        Link encap:Local Loopback  
          inet addr:  Mask:
          UP LOOPBACK RUNNING  MTU:16436  Metric:1
          RX packets:0 errors:0 dropped:0 overruns:0 frame:0
          TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0 
          RX bytes:0 (0.0 b)  TX bytes:0 (0.0 b)

[root@localhost ~]# ifconfig eth0 up
Setting slip line discipline: Invalid argument
uml_net : waitpid process 27924 failed, errno = 10
get_ifname failed, err = 22
SIOCSIFFLAGS: Invalid argument
[root@localhost ~]# route
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface     *          U     0      0        0 lo

[from the docs, 2 steps to configure the interface for slip, albeit not explicitly given.]

[root@localhost ~]# cat /etc/sysconfig/network-scripts/ifcfg-eth0 

[root@localhost ~]# halt

#HOST ifconfig sl0 pointopoint up
SIOCSIFADDR: No such device
sl0: unknown interface: No such device
SIOCSIFDSTADDR: No such device
sl0: unknown interface: No such device
sl0: unknown interface: No such device

Best guess at this point is I'm missing something here to create the virtual slip network interface, and possibly the other portion is failing because it is not present, but the errors are not so helpful.  "Setting slip line discipline: Invalid argument" "uml_net : waitpid process 27924 failed, errno = 10" "get_ifname failed, err = 22" "SIOCSIFFLAGS: Invalid argument" - sounds like the guest side (or echoing the host side, vague) was unable to find the slip interface.