I just bricked (hung at apple logo) my phone after installing some Cydia apps on my iPhone 4. I've always had my iPhone's jailbroken, and have never had any problems. After I had installed Wi-Fi sync, quick reply, and notifier, my 3G logo wasn't appearing so I did a network setting reset and now it's hung at the apple logo for over an hour, so I'm not sure which app it was, but just be sure to check for OS 4 compatability before you install apps. I'm at work so I can't restore until I get home, but everything should go back to normal after doing a DFU restore. No biggie!