Public service announcement:
I was having problems with php as I tried to set up websites on dreamhost before transfering DNS to DH. My idea was to get all the sites working perfectly on DH before I tear down the old server, so that there would be no loss of availability in the transition.
Normally this can be accomplished by locally over-riding DNS on the client side to point to the new host; then it is possible to test and interact with the site to make sure it's bahaving properly.
Though this works for standard html, php seems to not execute properly in this context. I would receive the following errors when attempting to execute any php code at all (even phpinfo()):
Premature end of script headers: /dh/cgi-system/php.cgi
I was escallated to level two support before finding this all out on my own; while waiting for level2 response, one of my other domains finished being xfered to DH (so DH now has authoritative DNS for this domain). PHP now executes cleanly, even though the DNS changes have not propogated beyond DH yet.