Expanding on ETHeREAL's answer (no need to upvote).
If we ultimately want to know approximately how many people hold both ETH and DAO, there are some flaws with any answer we come up with, which is why I asked the question in the way I did. However, it's worth noting:
1 account != 1 person.
Percentage of ETH holders that are also DAO holders may be significantly higher that data shows as people may not have wanted to expose 100% of their ETH holdings to an account that held DAO. (thx auryn_macmillan)
ETH accounts with teeny, tiny amounts are included in dataset but if we assume these are accounts that once held ETH, and then moved to a new account but ultimately miscalculated gas, then we should exclude these from our dataset as well. It wasn't until very recently that Mist added the "Send All" functionality. If The DAO had been around longer, or it was equally hard to "Send All DAO" as it is/was to "Send All ETH", then it might be okay. This also includes the
1 wei accounts which are often used for verification or signing, not an actual holding account that would ever purchase DAO.
I'm making arbitrary cutoffs just too see what that range is. We will assume anything less than that balance is no longer an active account that could have ever invested in The DAO, or is accounted for in another account. (thx BokkyPooBah)
balance > 0.0001ETH = 14.43%
= .144304596 = 14.43%
balances > 0.01 ETH = 19.78%
22873 / (109330 + 6289) = 19.78%
balances > 0.1 ETH = 25.76%
22873 / (82482 + 6289) = 25.76%
balances > 1 ETH = 35.59%
22873 / (57975 + 6289) = 35.59%