Hardware constraints might be a limiting factor if you're going to monitor a large amount of traffic. You also only get one NIC by default, so you'll have to perform monitoring and management on the same port. It'll probably be fine for playing around with at home or in a small office though.

If you're just looking to do IDS activities in a small form factor, check out the Soekris boards: http://soekris.com/

They go up to 1.6Ghz/2GB/4 Ethernet ports, but they're obviously much more expensive than a Raspberry Pi.