I've long been puzzled by the official advice on upgrading Drupal

It basically says you should delete everything, unpack a new Drupal version, and replace your customisations.

I like to use version control to manage my site.

My current project uses the pressflow fork of Drupal, I downloaded the current and new versions of this, added them to my repository and then upgrading my working copy is as easy as

This merges in all the changes (including file/directory additions/deletes).

Then I play around with the newly upgraded site, have a look at the code that changed, check all is OK and when I'm happy - commit the changes.

If I later find a bug that might be related to an upgrade I've got the changes ready via the usual tools (annotate, log, diff).

Even better - this retains any changes I've made to core without any need to keep a list of patches. If the core update contains a fix I've applied locally then I'll get a conflict which I can resolve.