On my greg-willis.com website I've got the Apache Solr Search Integration module set up and working as my search and 'related content' engine. In my recent upgrade frenzy, I upgraded to the 6.x-1.0-rc1 version and promptly busted it. Since I became busy with projects that actually paid money, I disabled the module and life went on. But I had a break today and figured it out.

The Error:
After I upgraded when I went to admin/settings/apachesolr I received a php error that referenced a missing SolrPhpClient/Apache/Solr/Service.php.

The Solution:
I found the solution on an install page, Step 4 of 9 Installing “Apache Solr Search Integration” Drupal module (surprisingly enough). In terminal, enter:
svn checkout -r6 http://solr-php-client.googlecode.com/svn/trunk/ SolrPhpClient
This will install the needed SolrPhpClient/Apache/Solr/Service.php file. Hope this helps.