I used a razor when I moved into my house. Took a while, but it did get it off. I actually used one of those paint scrapers that you slide a one-sided razor blade into. Your hardware store will have one.
You can use Easy-Off Glass Cooktop cleaner. http://www.easyoff.us/faq.shtml#section1
I used the original heavy duty stuff, but made sure it didnt come into contact with any aluminium. (ie remove the heater units and clean them elsewhere.)
Two things: Cerama Bryte cleaner will take off anything I've ever thrown at it:
And as elfcook says, a straight razor-blade scraper will work too, in conjunction with a cleaner like Windex. The Cerama Bryte is the gentlest and safest, though.
if the cleaner & "approved" scrubby sponge do not work, there is another way - you just have to be careful. Take a razor blade & gently scrape the gunk, holding the blade at an angle. This was suggested in a manual for a stove I used to have. I even once picked up a glass top cleaning "kit" of some sort that had a plastic handled razor blade for this purpose.