Bug fix update of some sort. I can't find release notes on the website, but maybe the App Store has something.

What's New in Version 4.0.1

- Improved Assistant editor logic when switching among different file types
- Fixed a bug in "Install Xcode.app" that hangs at 99% complete, never finishing
- Fixed a bug that prevented indexing of some projects
- Fixed a bug related to nil settings in the Core Data model editor
- Fixed a bug that prevented automatic download of iOS documentation
- Fixed a bug in LLVM GCC 4.2 and LLVM compiler 2.0 for iOS projects
- Additional bug fixes and stability improvements

Xcode 3.2.2 adds support for developing iPad and Universal iPad/iPhone apps • New menu option: Project -> Upgrade Current Target for iPad... • To configure Universal apps that run on both iPad and iPhone, in Build Settings:
- set Base SDK in the Architectures section to “iPhone SDK 3.2” - set iPhone OS Deployment Target to iPhone OS 3.1.3 or earlier
• For iPad-only development, use iPhone SDK 3.2 and target iPhone OS 3.2 • For iPhone-only, use iPhone SDK 3.1.3 and target iPhone OS 3.1.3 or earlier • Be sure to set Targeted Device Family to iPad, iPhone, or iPhone/iPad
http://adcdownload.apple.com/iphone/iphone_sdk_3.2__final/final_about_xcode_3.2.2_snow_leopard.pdf

Xcode 3.2 developer tools for Mac OS X 10.6 has the following improvements:
• Static code analysis is fully integrated within the Xcode IDE via the Build and Analyze option under the Build menu or via custom build settings
• New Build Results window collects warnings and errors in a persistent log
• GCC 4.2 is the primary system compiler for the 10.6 SDK
• The next-generation LLVM compiler is included using two different front ends - the Clang compiler is a new, leading-edge front end that also provides the basis for the new static analyzer. For maximum compatibility, the GCC LLVM compiler utilizes the LLVM back-end with the GCC 4.2 front-end parser.
• New template chooser and project assistants • New opening window for quick access to recent projects

• The assistant interface has been completely revamped, making it easier to
create "New Project", "New Target", and "New File" resources. This release of
Xcode also adds new assistant templates for both Mac OS X and iPhone OS
applications.
• A new Overview toolbar item lets you see and set the project's active Target,
Configuration, Executable, Architecture, and SDK with a single control.
• It is now possible to set a global "override" SDK as the Active SDK, making it
easy to test the build of your entire project against a different platform.
• New interface shortcuts have been added to the Build Setting inspectors
making it easy to set the desired compiler, architectures, and SDKs
• You can now add Libraries and Frameworks directly into the Target window and
relate them to the project's SDK.
• The Xcode UI now allows you to designate a library or framework as weak-
linked, to support building with newer SDKs while targeting older OS versions.
• New "Edit all in scope" option within the Xcode source editor makes it easy to
update all instances of the selected symbol within the current scope of code.
• You can now have conditional build settings assigned for any combination of
architecture and SDK.
• LLVM GCC 4.2 is a new optional compiler based on the LLVM.org open source
project. LLVM GCC 4.2 provides an LLVM-based back-end optimizer using the
GCC 4.2 front-end parser. This compiler is both source and binary compatible
with GCC 4.2 and requires the Mac OS X 10.5 SDK or "Current OS" SDK.
• GCC 4.2 is a new optional compiler with stricter C and C++ language
enforcement, security enhancements, performance optimizations, and bug fixes
over GCC 4.0. Requires the Mac OS X 10.5 SDK or "Current OS" SDK

• SDK support for targeting non-Mac OS X platforms, including iPhone OS SDK.
• GCC 4.2 & LLVM GCC 4.2 optional compilers for use with Mac OS X 10.5 SDK
• Updated assistants to create new projects, targets, and source files
• Toolbar uses a single popup to choose platform, target, and debug/release
• Integrated SCM support now works with Subversion 1.5

"Xcode 3.1 is an update release of the developer tools for Mac OS X, and is the same version included with the iPhone SDK. This release provides additional GCC and LLVM compiler options, general bug fixes, and must be installed on Leopard, Mac OS X 10.5.0 and higher. Xcode 3.1 defaults to upgrading Xcode 3.0, but may optionally be installed along side existing Xcode installations. See the accompanying release notes for detailed installation instructions, known issues, and security advisories."

Xcode 2.4.1 is the latest release of Apple Computer's integrated development environment for Mac OS X. This release addresses many known issues and is a recommended upgrade for all Xcode users of versions 2.2.1 or earlier. Please choose Show Older Release Notes from the Help menu to see the release notes for previous releases of Xcode. Additionally, the complete list of Xcode and related tools release notes can be found from within Xcode by choosing Help -> Xcode Help from the menu bar, and selecting the "Release Notes" bookmark item or look in the /Developer/ADC Reference Library/releasenotes folder.

The Xcode 2.4 release provides overall stability improvement and adds support for 64-bit Intel based Mac computers. It is recommended that all Xcode users install this update.