You want to listen in from a hub without showing up on the network.
You can connect Snort to the hub using a receive-only Ethernet cable.
To make the cable, take a normal Ethernet cable and carefully split it somewhere along its length. Carefully extract the pin-one line (on most normal Ethernet cables, this will be white with an orange stripe), snip the line, and solder in a 23 pF capacitor.
You can turn off the IP address using ifconfig, but shutting down the IP address is only one step. It is possible to make a network card respond to protocols below the IP stack level. Protocols such as ARP and ICMP do not cease to function just because you have the IP address turned off; this could allow a skilled intruder to detect an otherwise hidden IDS.
If you are trying to keep things simple, remember that an IP address is not the only way to detect an IDS. Other aspects of the system may show an IDS, such as network traffic sending alerts, names of systems in DNS that either include IDS in the name or appear suspect, and the behavior of active response systems that indicate that something is listening.
Snort online documentation, "IDS Deployment Guides" (http://www.snort.org/docs/)
Invisibly Sniffing Between Two Network Points
Installing Snort from Source on Unix
Logging to a File Quickly
How to Build Rules
Detecting Stateless Attacks and Stream Reassembly
Managing Snort Sensors
Generating Statistical Output from Snort Logs
Monitoring Network Performance