I've been told that there was an FAQ for 5th that allowed you to join ICs to units before deployment, then place them on the table. But, that FAQ was removed with the release of 6th edition.
Potential gimmick: attach Illic Nightspear to a unit of 10 Wraithguard all with D-scythes and infiltrate the unit into your opponents face.
Went back and looked at the BRB and Codex, and this is what I'm seeing.
Doesn't work because you can only attach an IC to a unit by placing it on the table within coherency, or by putting him reserves with the unit (page 39). The wraithguard are required to deploy on the table or be committed to reserve before the Illic gets a chance to join them, so they can never benefit from his infiltrate rule (other than using it to outflank) since the scytheguard must deploy normally and Illic deploys last as an infiltrator (page 121).
Units don't join each other in the infiltrator pool either, so while Illic and Karandras or a bunch of Scorpions in a Serpent could complete their deployment in coherency, Karandras/Scorpions would be restricted to the normal 12/18 inch bubble from enemy units before being joined by Illic or being placed on the table.
I'm not seeing how to confer Illic's special infiltration to a unit before deployment.
On the other hand, it would be really cool to be wrong and find a RAW way to be able to sneak in a unit by conferring infiltrate to it, or get other infiltrators into even better positions by taking advantage of Illic's ability to bypass the normal 12/18" restrictions.