Jailbroke this morning, and was able to successfully install an app from the appstore as expected.
What are the essential apps you've gotten through Cydia?
Right now, I have:
SBSettings and Install0us are the most important.
To get Install0us to work, you need AppSync for OS 4.0.
I also installed CyDelete and iPowerHour (Yes, the drinking game).
There is a hack for My3G and MyWifi, which allow you to trick your device into thinking it's on WiFi when it's on 3G, and to tether respectively. I'm not so sure about them, though. I may hold off until a legit free version of these are released, which I'm sure they will be.
Once I get this figured out, I may PM you about getting the AppStore.app off you, so maybe I can restore whatever's corrupt with mine.
I still need to do a little research first though.