Some victims reported that they have unique password for XBL, and some of them don't have EA accounts prior to the hack. It may be due to a number of techniques but XBL should develop capability to detect and minimize them further.
How seriously are you taking online posters in regards to their memory of PSW integrity and account registration?
As for that later point, I am pretty sure MS has a fairly robust strategy for hacking as well as constantly updating it (everyone I know who works with MS says they are extremely security oriented) *but* there is no way you are going to be able to prevent advanced data mining approaches from compromising individual attacks. Sorry, when you have banks, online services, and social networks (which have HORRIBLE security and freely gleaned data provided by the users themselves) tossing tens of millions of user account info right into the general data pile it is impossible.
Yes, MS has to do their part (and they still may get hacked, it isn't impossible) but users are a prime source of data. This is why phishing is so popular--because you don't even have to data mine as many users will GIVE you the data.
We will see how this shakes out but I am sure even if users are to blame it will still be said to be MS's fault by some.
As for me? I missed out on all those free PS3 games so I could use a couple good new games for free on the 360 side