• Big improvements in stability. Since the last version 100s of bugs were fixed.
• Feature complete - Although some of the 2.5 targets have been postponed, such as multi-window support showing multiple scenes, a full RNA data level dependency graph, or radial menus.
• Changes – If you’re used to the old Blender, Blender 2.5 may seem quite different at first. Be prepared to read a bit about this, how to reconfigure things, and learn to use the new built-in 2.5 search functionality!

Source:

http://www.blender.org/development/release-logs/blender-258/

Official text:

Changes since v0.8RC4:
- exporter fixes and packaging

Other changes since v0.7.1:
- New GUI options to save render on demand (most file types are supported), with or without separate light groups, statistics overlay, …
- new glossytranslucent material, initially developed for better leaves rendering
- film response curves to emulate traditional cameras color response (some curve are for black&white films too)
- GPU acceleration for path tracing when sampling one light at a time
- render queue to process several files in batch mode from the GUI
- support for split vertices in mesh subdivision (mesh will no longer open up in the presence of UV/normal seams when subdivided)
- new light groups color handling for better accuracy when adjusting light color
- outlier rejections (this will eat slightly more memory and slow the render a bit, but the noise level will be reduced significantly)
- finally usable exphotonmap integrator
- new band texture to enable complex gradients
- new volume scattering feature atop the volumetric absorption introduced in v0.7 (only homogeneous volumes for now)
- much improved ply mesh loader (supports quads, texture coordinates, subdivision, …) and export to ply for various exporters instead of lux native (faster export and loading)
- on the fly microdisplacement to allow very high subdivision depths without any memory impact
- various fixes/improvements to the pylux library (should help Blender 2.5 integration, now using python 3.2 to follow Blender change)
- proper volume integrator support warning: behaviour change, the “single” volume integrator will only do single scattering (good for aerial effects), there’s a new “multi” volume integrator for multiple scattering (good for SSS)
- improved mesh subdivision, the new “bool dmnormalsplit” option allows to create edges at shading normal splits so that hard edges are preserved by the subdivision
- new “multimix” texture to create arbitrary mixtures of textures
- removal of the “highcontrast” tonemapper
- photon shooting can now be aborted by stopping the render
- film output file path can now be overriden in luxconsole with the -o/–output option
- EXR output is now always linear (no gamma correction) even when tonemapped and post processed
- bidirectional integrator fixes
- volumetrics fixes
- render queue fixes
- nextwork rendering fixes
- the alpha channel is now exported if requested when saving from the GUI
- “environment” camera is no longer horizontally flipped and points in the right direction
- camera response presets so you don’t need to keep the files around for most camera film types
- fixes for IPv6 networking support
- fix an issue when using IES profiles on area lights
- better handling of filenames across platforms (\ vs / separator issue)
- ability to load STL meshes
- new advanced tab in the GUI exposing various informations on the render configuration
- ability to use local coordinates, UV coordinates or normal coordinates for Blender and PBRT textures
- fixed light intensity when using an imagemap or bilerp to texture an area light
- new stop button behaviour in the GUI, the render can now be restarted, a new menu option allows a full stop of the render engine
- ability to embed file data inside LuxRender files
- new option for EXR output to output straight colors even with alpha channel (the standard requires that the color is always multiplied by the alpha value)
- ability to load and save tonemapping and post effects parameters in a file
- improve luxconsole statistics output
- make flm file writing more robust
- new color clamping scheme selector in the GUI
- check LuxRender network protocol version when doing network rendering
- fix alpha output when saving from the GUI
- new “solo” (appears as a + or – in the light group title bar) light group selector to disable all light groups but the selected one
- support IES files with comma separators instead of spaces
- add a centering option to the STL mesh loader
- prevent black splotches when using carpaint with the photon mapper
- various fixes and improvements all around

Source: Luxrender.net

Blender 2.57b was released on Wednesday, April 27th 2011.

Best testing only happens after a real release! With nearly 100 great fixes in the code, Blender team released another update for 2.57.

Source: Blender.org

- improve luxconsole statistics output
- make flm file writing more robust
- properly initialize the clamp selector in the GUI
- check LuxRender network protocol version when doing network rendering
- make more external data embeddable in the scene files
- fix alpha output when saving from the GUI
- new “solo” (appears as a + or – in the light group title bar) light group selector to disable all light groups but the selected one
- support IES files with comma separators instead of spaces
- add a centering option to the STL mesh loader
- prevent black splotches when using carpaint with the photon mapper
- fix volume regions scattering (old volume system)
- various fixes and tweaks

Source: Luxrender.net