I was under the impression that the actual date that we process the e-filing was the date the taxpayer is credited with filing their return, but it seems that was not the case for this taxpayer -- they've received a notice with a late-filing penalty. IRS acknowledgements were obviously considerably delayed on the 15th this year, so this particular return wasn't even sent to IL until 10/16. 

We're a West Coast firm and file almost no IL returns, and none of the states we regularly file have ever (to our knowledge) "behaved" this way in these circumstances. I will be responding to the notice accordingly, but am posting here in case anyone has any IL-specific guidance and perhaps to provide a heads up to others in case this was not a one-off.