The Windows Installer 3.0 Technologies beta newsgroups have opened and the beta has begun, although its not listed on Betaplace yet except in the contact info section.
Improvements and new features:
Compatibility - Packages and patches authored for MSI 2.0 are fully supported on the MSI 3.0 engine. By just installing MSI 3.0 users will be able to benefit from its many servicing improvements. MSI 3.0 patches have additional data tables which are necessary to completely exploit the new features. It is important to note that MSI3.0 patches can be installed by MSI 2.0 – the engine will just ignore the new tables and the associated functionality.
Patch Removal – MSI 3.0 provides native support to remove small and minor updates (patches). Any patch installed by the MSI 3.0 engine is eligible for removal. The removal of a patch is the functional equivalent of never installing the patch. The state (files, regkeys, etc...) of the product when a patch (or patches) is installed and removed is identical to its state where the patch (or patches) was never installed. Patches can be removed in any order. Patches are removed using the command line or the calling the MsiRemovePatches API.
Sequencing – The new MSI 3.0 patch sequence table enables authors to provide explicit instructions for the order in which updates should be applied on target machines. Updates will be applied to target products in a consistent and predictable order regardless of the order in which they are physically provided to the target machine. By adding the patch sequence table authors can reliably update un-versioned files. Patches without the sequence table will be applied in the order in which they are provided to the engine.
News source: Betaplace