When you setup a domain to use GMail/Google-Apps via DreamHost's Panel, the correct MX and CNAME (for sites, mail, calendar) are setup automatically by DreamHost. Since all incoming/outgoing mail is handled by Google's servers, there is no need for reverse-DNS entries on your domain.
SPF, However, is not automatically setup. So you will need to manually setup an SPF TXT record for your domain, using an include statement to specify that your mail will come from Google's mail servers. For example, I use:
v=spf1 a include:aspmx.googlemail.com ~all
Although GMail uses Domain Keys for email@example.com addresses, Google Apps for Domains does not appear to support Domain Keys -- at least that is the impression I got when I was setting up my domain. If Domain Keys are very important to you, you should research it some more.