In answer to a support request during the beta period, Nathaniel acknowledged a bug in SA filtering for mail from within DreamHost accounts. His answer to my question was more detailed, but the bottom line was this:
1) If the smtp server for the sending domain is configured to deliver mail for the receiving domain, the email is NOT filtered.
2) Since they use several different mail clusters, this results in SOME mail between DreamHost-ed domains being filtered and some not.
3) Because DreamHost has an aggressive spam policy, this isn't a big concern. If you ever get spam and it's from a DreamHost customer, let them know, and they will deal with the person immediately.