I would recommend you check this post/blog out http://homeenergypros.lbl.gov/profiles/blogs/codes-ignition-barrier... to get a better handle on the who, what, when, options, etc...
With that said, I don't know of any in my area that check much less enforce it
I do not have a full cavity foaming rig, but when I do I sub the work out to a subcontractor. He carries a full cavity open cell spray foam product that is fire rated and does not need to have an ignition barrier on it. The product is Gaco Firestop I can not say that i am an expert in it but i have used it a few times in areas where the homeowner was not interested in dry walling and finishing the space. It cost a little more than common open celled spray foams but the lack of need to spend days on the site easily makes up for that.
Watch out for thermal barrier requirements in addition to your ignition barrier concerns. If a crawlspace is open or has access to a basement their may be code enforcement issues with some crawlspaces. To deal with this the choice is to not use foam or make sure there is a thermal barrier seperating the basement and crawlspace. Some foam does get used on ductwork but it must be the fire rated formula. The fire rated stuff looks identical to the standard foam most use. You would need to get your hands on the technical data sheet (easy to find online). Similar foam but slightly different density, R value and cost. Hope that helps you.
When we use this product we place a laminated technical data sheet in the given area and give the home owner a copy to keep with their receipt to show any one that may ask what the product is and that it does not need to be covered with a fire barrier. It can be deceptive and when with out the data sheets there would be no way to know that the foam is fire rated.
Cory is right, watch out for thermal barrier requirements as well. In it's simplest form if you can walk up to it on any given day it will require a thermal barrier. if it is harder to get to you will need an ignition barrier. The FS /SD numbers from the E-85 teat are not enough. Foam plastic has it's own section in the IRC.
The E-84 only test has was removed from the IRC between the 2003 & 2006 code cycle. The exceptions you are speaking of are for THERMAL barriers not ignition! You need an ignition barrier unless the specific foam you are using has passed either the NFPA 286, FM 4880, UL 1040, UL 723 or UL 1715 test. NOT E-84. This test (corner crib) has been modified in AC 377 for ignition barrier only. Other than passing one of these tests the foam must be covered by at minimum a ignition barrier. Please read IRC 2009 R316.5.3 & R316316.5.4 Both section require attics or crawl spaces with limited access to be protected by an ignition barrier.
The only true exception is for sill plates and headers IRC R316.5.11
I do not see how this could be "interpreted" any other way that written.