Before 'major' 0.6.5 we improved Web Optimizer a bit with the following things.

Added cache re-generation instead of cache clean up. Now all files are in cache cleaned up, then Web Optimizer tries to re-generate them. While cache files are being generated website doesn't use Web Optimizer - to prevent CPU overload with clean cache. Only after we have all files ready to be served, we can completely activate Web Optimizer
Added 5 cache file sets generation on options' save. The approach described above can be used when options are saved.
Added option to force deflate over gzip for IE6/7. It seems IE6/7 plays with deflate page encoding a bit better than with gzip. So you can now force Web Optimizer to use this encoding for these browsers.
Added options for better placement of Web Optimizer stamp and link. There was a complete post about Web Optimizer stamp placement.
Added IfModule for all .htaccess rules. Since there were a few complaints about broken websites after .htaccess usage it's completely safer to add IfModule for all sections. This will block normal behavior of Web Optimizer in case of incorrect server side functionality detection. But won't break the whole website.
Improved static assets proxy for images. Added graceful degradation to redirect to the resource and fixed a few issues.
Improved mhtml behavior for Vista. Just disabled with a new set of cache files.
Improved CSS Sprites behavior for IE6. Added BackgroundImageCache directive for this browser to prevent CSS Sprites blinking.
Improved Apache modules detection on CGI environments.
Improved Joomla! plugin / mambot behavior on a few environments.
Improved dynamic styles' loading on DOMready event.
Fixed issues with performance / gzip / footer options incompatibility.