My understanding is that this is an ongoing program, and does not end at the end of 2008. It is true that the state only mails checks once a year, usually in the spring. Applications made during any calendar year are paid in the spring. If you do delay, you may miss the 2009 spring payment. The state will look for receipts for the equipment, and will look for several pictures of the electrical equipment in place. They have not been real picky about the photos, and do not require an inspection or proof of actual range for the rebate. I know they accepted a car with pictures of motor in place, and batteries in place, with the batteries wired, but did not notice that the main fuse was not yet in place, or a couple of other smaller things. That car had it's pictures taken after the installation of the main items, but before every last thing was connected, and they approved the rebate on that car.
Licensing is a different issue entirely.
If you want the rebate in Spring of 2009, get started now. Submit your application with receipts and pictures and W-9 form before the end of the year. See the instructions at:
http://www.illinoisgreenfleets.org/fuels/ and scroll down to section 3. That will give you the exact procedure to get the rebate.
The State of Illinois has funded this as an ongoing program, and of course, could withdraw the program at any time in the interest of budget limitations. I have not read any statements about ending the program, but government programs are subject to change.