Just a note to hopefully save some others some time.

If you have a drush makefile that includes the apachesolr module, the module ships with it's own makefile that will fetch the proper SolrPHPClient libs for you. This is a good thing :)

However, due to this issue in drush_make: http://drupal.org/node/1159548 since there's nothing else in the makefile, drush make will bomb out with "No core project specified". Dmitri has committed the fix into drush_make 6.x-2.x, but this bugfix won't show up until drush make 6.x-2.3 is released. Using drush_make 6.x-2.x (dev) will fix the issue.

I'm marking this as won't fix right out of the gate, because there's nothing for the apachesolr owners to do here.