If you can nail a good user-facing experience and a strong back-end, I fully support this over Bazaar (and the other one that is even worse, Deepshining?). It seems developers really don't care about Bazaar, it's too focused on tweaks and even then, the hackers don't upload the latest versions to Bazaar.
Installous is just a way for ****ty iphone owners to steal apps instead of paying for them.
I hope this is more Cydia than Installous. One of the great things about Cydia is you can still sell your product, you just don't have to put up with whatever restrictions Apple has on their store. For example, if they decided to permanently pull Airfoil Speakers Touch, Rogue Amoeba would still be able to offer their app, albeit to a much smaller audience.
@OP, one problem I see: you can't install XAPs from the phone on a developer unlocked phone. I believe it requires at least Root Tools, or full unlock. If your solution is to distribute the XAP to a PC and let users deploy to the phone, this rules out paying for apps. The way it works with full unlock (and the necessary tools): the XAP is downloaded to the phone, deployed then immediately deleted.