Apple released iOS 4.1 to developers this week, but the fix that the company promised to "fix" the signal bars is now out for those with an iPhone 3G, 3GS and 4 on the eve of the press conference that's happening on Friday morning.
This update brings it up to just 4.0.1. The small bump in version numbering implies that this is just a small tweak in how signal is calculated and presented in bar form.
Funnily enough, one of the most noticeable effects of the tweak is that now the leftmost three bars have grown in height. See the image from Engadget (old on top, new on bottom):
For all of you who depend on a carrier unlock, beware that 4.0.1 brings with it a baseband update for the 3G and 3GS that it's not yet unlockable, so don't play around with that update button in iTunes just yet.
Apple has also released an update for the iPad to iOS 3.2.1, giving it the following improvements: