Please see these pieces of code - might be helpful

#434944: Allow minor upgrades of drupal core

especially - http://drupal.org/node/434944#comment-1931644

my script if I remember well is quite better than the next comment's script there...

Please diff my version there with the most recent one I attach - I do not know if I changed it afterwards...

Script used many times without problems - works from a drupal's root dir and with no args checks drupal core. With a package's name (usually coincides with a module's name) checks a specific module... Works by downloading the module/core to /tmp

Please tell me if I help you or your solution is better.

This code here is a bash fast prototype that works... The ultimate goal is to "PHP" it and "drush" it ...