After yesterdays private release of iOS 4.1 to Apple Developers, Apple has now released iOS 4.0.1 to the general public.
The 4.0.1 update is for iPhone 3G, iPhone 3GS, and iPhone 4. This update includes the new, improved formula for calculating signal strength, though no real change has been observed.
A 3.2.1 update for iPad has also be released fixing Wi-Fi issues among other things:
Fixed an issue that could prevent copy and paste of single-page PDF attachments in MailJailbreakers beware, don't update your device until an official jailbreak is out.
Addressed an issue that could cause video playback to freeze
Improved reliability of video-out when using iPad Dock Connector to VGA Adapter
Added Bing as an option for Safari's search field