With my iPhone 3G jailbroken with iOS 4.0, lots of other users have been experienced cell phone reception trouble. I tried it myself and covered the left side and saw my signal bars drop by 4. I've read around the internet and its rumoured that 4.01 was supposed to drop today to fix the antenna issues. I'd assume before Friday, there's going to be a fix out there. I, myself, didn't have too much trouble with getting a signal. Where I was reading on Slashdot, it was said that even if the signal is 5% or 100%, you're going to get the same call quality.
I haven't got BiteSMS to work on 4.0 yet, nor five column springboard (fcsb) or Infinidock. A couple Winterboard themes don't work either, such as custom Wifi and signal bar icons. Cycorder doesn't work either. Hopefully, these'll be fixed soon so I can get 4.0 running like I had 3.1.3.
Also, I was thinking about rejailbreaking my iPhone to disable multitasking. Apparently, Backgrounder uses more resources than native multitasking (duh). If two apps are running at the same time and one requires more resources than the other, the one running in the background will be terminated leaving the foreground application with more memory.