Starting January 1, 2013 you will be able to purchase, transfer, make, possess, and use a SBR in IL. At first glance it appears to be very limited and only apply those with a valid and active military re-enacting group membership to use them for military re-enacting. But there is an interesting exception, those with a Valid C & R license can also own them.
The Law is poorly written and leaves many questions unanswered. A Gun Trust cannot have a C&R license but a Trustee of a Gun Trust Can have a C&R license. The question is will ATF understand allow A Trust to purchase a SBR if all Illinois based Trustee's (the authorized users) have a C & R License as required under IL law. If not you will still need a CLEO sign off because you can't have a C&R within a Trust.
Also once you modify a C&R firearm it no longer retains its C&R status but the Illinois law is different than similar laws in CA because if you have a C&R license, you can buy any SBR not just a C&R SBR.
In Summary, SBR's are legal in IL if you have a C&R regardless of whether the SBR is an SBR. Can you use a Trust? That's up in the air at this time. You should be able to if every Trustee in the state has a C&R but we do not know what the ATF's position will be.