Running into problems un-jailbreaking

I'd like to un-jailbreak my iPad in order to install iOS 4.2. Right now, I'm running a jailbroken version 3.2. Every time I try to "restore" the device, i get an error message that says "The iPad could not be restored. This device isn't eligible for the requested build" after iTunes tries to verify iPad restore with Apple. Has anyone run into this problem before? How can I unjailbreak the iPad?

Can I simply press "update" in iTunes? to update the device?

Thanks!

UPDATE:

I have not messed around with any advanced settings in Cydia (and have not used Cydia for several months).

 

Thanks for clearing this up. I'm essentially a rookie with jailbreaking, as I haven't done it since 08 on my original iPhone.

Thanks again!