Switched a Drupal 7 site over to dreamhost from mediatemple. PHP wasn’t working, so I tried to isolate the problem. My suspicion was that the stock .htaccess file that comes with Drupal 7 was chucking a wobbly, so I temporarily renamed it and I was right. After changing the filename back, I added this line to the top of the .htaccess file, which fixed the problem:
AddHandler php5-script .php