The Illinois Department of Revenue recently announced it is in the final stages of internal front-end testing and hopes to finish up this weekend.  If that happens, it could begin processing Illinois e-filed tax returns as early as Monday, February 2, 2015.
Preparers should expect delays in acknowledgement turnaround time due to the expected volume.

The somewhat unusual delay appears related to new identity theft prevention measures, based on this message on the Illinois Dept. of Revenue website:

We are not accepting electronically filed 2014 tax returns at this time as we continue to test new identity theft prevention measures.  Check back here for the most up-to-date information for these services.

Check www.tax.illinois.gov for the latest updates.