Apple has scheduled an iPhone 4 press conference
for Friday morning, so what better day to release iOS 4.0.1, which reportedly fixes the signal bars display on the iPhone, ensuring you know exactly when the "death grip" will cause you to drop a call. Aside from that, iOS 4.0.1 does nothing.
In fact, the release notes displayed (above) when you update your phone says it all: "Improves the formula to determine how many bars of signal strength to display." This is exactly what Apple said iOS 4.0.1 would do, and although they said it would "fix" the signal reception issue seen when the iPhone 4 is held.
You can rest assured that the iOS 4.0.1 update will be addressed at tomorrow's press conference, as a "positive" with respect to the "death grip" issue.