Funny, because Cydia is a concept that was created as a response to Apple's restrictive controls over which apps enter the App Store.
Cydia is possible because the App Store holds a monopoly over iOS apps. In order to get apps not approved by the App Store, Cydia requires jailbreaking.
The thing is, the Mac App Store is nowhere close to holding such controls, nor does Apple want to hold such controls over the Mac applications market. Cydia was only possible because iOS can be jailbroken; the concept of jailbreaks do not apply to Macs.
With Mac OS X, you have freedom to get whatever app you like from any source. So, there's no rationale for Saurik to develop a Cydia for Mac, when you could just simply visit the developer website and get the application there yourself.
Therefore, Saurik probably won't create a Cydia for Mac; it's pointless. And we don't know if such a thing exists yet. It's best if you email Saurik yourself, but he'll probably say it's not happening.