It looks like Ozcoin is processing all transactions, Bitparking is passing along no transactions, and Deepbit has a transaction fee threshold that prunes lower fee and/or free transactions from their blocks.
The question then, is, does this make a difference to them? Are deepbit (and to a lesser extent bitparking) keeping a low orphan rate or is this barking up the wrong tree? I still get the feeling we're missing the underlying cause for this since it predates the abrupt rise in the number of transactions.