Can I still use Cydia after I remove the Jailbreak from my Ipod Touch?

[Blacklist 02905]

I recently downgraded from 4.2 to 4.0 and jailbroke it/installed Cydia, can I remove the Jailbreak, update to 4.2, and still be able to use Cydia?
Edit: So that means I have to stay at 4.0 forever? >_>