IDS deployment outside FW?

Dear List

I have moved into an organization that has two RealSecure Network Sensors 
and a network architecture that is VLANd/DMZd to where localized deployment 
to capture traffic would require 8 to 12 sensors to avoid bridging loops.

The cheapest/simplest option (without deploying SNORT/Prelude, etc - the 
organization wants to remain on a single application architecture where 
possible) is to place the two sensors outside of the firewall.

I understand that this means:
The sensors will be in hostile territory and need to be maintained to a very 
high degree
There will be an operations overhead of dealing with all of the noise that 
would normally be filtered by a firewall

Does anyone have experience of doing this?
Are there any other issues that I have not considered?

Chris

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