Core updates are done by the webserver downloading the latest version of WP from wordpress.org (or lang.wordpress.org), unpacked and updated.
I don't think that the webserver should download the whole package. Changed files are easily tracked by Trac. For example:
This page contains a list of changed files between the two versions and if that list could be generated, zip files with the files in the list should also be generated (e.g. wp-3.0.4to3.0.5.zip or wp-3.0.1to3.0.5-tr_TR.zip).
This should increase the performance during core updates. Smaller files would be downloaded quicker, unpacked quicker and updated quicker. Right?
PS: Please don't say that "you should create an issue on Trac", I really tried but couldn't understand how that system works :)