The upgrading mechanism in recent WordPress version downloads an incremental upgrade zip file. I couldn’t find anything online to get the urls, so I dug into the code and I’m posting here.
The upgrade from 3.3.0 to 3.3.1 was in the file wordpress-3.3.1-partial-0.zip. I’ll try to remember to update this post as future versions come out. I’m not sure of the exact file format. I think it’s -partial-0/1/2 to mark how many minor versions back the upgrade goes.
Personally, I use these files for minor upgrades because it’s faster, but I use the full download file for a full version upgrade.