What's Coming in ArcGIS 9.3.1?
ArcGIS 9.3.1 is likely to improve the performance of dynamic map publishing and increases the sharing of geographic information.

1.High-Performance Dynamic Map Publishing

New faster rendering engine
Outperforms equivalent ArcIMS services.
Produces significantly better-looking maps.
Shortens map caching time.
Quicker, smoother zoom and pan.

New Map Optimization toolbar in ArcMap helps you tune-up your map documents before publishing to ArcGIS Server
Review and respond to errors, unsupported content, and warnings about items that will slow down your dynamic map services.
Preview your optimized map document and estimated rendering time.
Save your optimized map document to the new Map Server Document (.MSD)

2.Better Sharing of Geographic Information

Better sharing of layers through ArcGIS Online
Layers can be packaged and published through ArcGIS Online Web services.Layers can be found through a Web search.Contributors can organize and control access to the layers they share.

3.Enhanced Support for Java Developers

Ability to create the following extensions
custom geoprocessing tools
server object extensions (SOEs) for ArcGIS Server
class extensions for custom behavior
custom renderer's for customized rendering of data
plug-in data sources
custom layers

Support for Eclipse integrated development environment (IDE)
wizards to generate boilerplate code based on the developer's specification
autodeploy feature

4.Easy to Migrate

Upgrading to ArcGIS 9.3.1 does NOT require uninstalling ArcGIS 9.3.
Easy to migrate or work in mixed environments of ArcGIS 9.3 and ArcGIS 9.3.1 within the same organization since geodatabases, map documents, and APIs do not change.

5.Availability

ArcGIS 9.3.1 is scheduled to be available in Q2 2009.

http://www.esri.com/software/arcgis/whats-new/whats-coming.html