Previously you could install installer with cydia - likewise you could install cydia with installer!
I would hope that the DevTeam would use that method but if not you would have to remake your custom firmware with Installer added and restore and resync again.
To be honest although its great to unlock my 2G iPhones - Cydia has nothing to offer currently on my 3G!
Its just developer stuff and no apps at this time - but that will not neccessarily be any different with installer as the apps are not made to work with 2.0 yet!