Slackware 10.2 uses dhcpcd during boot to resolve network configurations, I don't know if this is the case with 9.1.
The thing with dhcpcd is that if it cannot aquire the configurations from the DHCP server within a time period (if it times out), it will bring the interface down for which it was ran. This is why you do not see eth0 up after boot, given that its not connected to a network.
Why is the loopback device not enough if you are not connected to a network?
Having an extra, essentially useless driver loaded will only waste memory.
But if you must have eth0 up, run:
modprobe <DRIVER_MODULE_FOR_YOUR_CARD> # Usually this step can be skipped
ifconfig eth0 up
ifconfig eth0 inet 10.0.0.1
eth0 now has the IP of 10.0.0.1 and should respond at that address (given that no firewalls running and route was automatically added - usually the case)
To bring eth0 down, run:
ifconfig eth0 down
/etc/rc.d/rc.inet1.conf (in Slackware 10.2 at least) has the boot-time configuration settings for eth0. You may modify it to make the start up script assign a static IP.