UPDATE: This bug on drupal.org has been fixed. I'll keep this article published, but it will no longer be listed on the front page.
Drupal.org is now (as of Halloween 2013) on D7. This is good news with improved features, especially in the issue queues. However, something happened with the upgrade that broke my drush make. There's something different in the project information on drupal.org. (See: https://github.com/drush-ops/drush/issues/234 if you'd like more info.)
This will all be worked out in time, but in the mean time I needed a fix. Drush make was not working for core downloads and I found that I could not even "drush dl" Drupal core. I came across this in the issues on Github. Add the following project information for Drupal core in your drush make file and the errors should go away:
core = 7.x projects[drupal][version] = 7.23 projects[drupal][type] = core projects[drupal][download][type] = get projects[drupal][download][url] = http://ftp.drupal.org/files/projects/drupal-7.23.tar.gz
I understand that some of these issues are being addressed and may already be fixed. One other thing I ran into was that the project type for themes is not recognised either. This caused my theme to be downloaded to sites/all/modules/contrib, rather than sites/all/themes I was able to fix it like so:
projects[boron][type] = theme
Note: I do recommend you have a look at Boron (http://drupal.org/projects/boron). It is a great HTML5 base theme that I've used as the base for a couple of responsive sites, along with SASS.