[mukyu]

The current DMCA exceptions granted by the Copyright Office[1] do allow you to circumvent CSS, but only for very specific educational or such use. Making a personal backup copy does not fit the criteria and thus is illegal, even though that is is normally allowed under just copyright.

[1] http://www.copyright.gov/1201/2010/

[Natsu]

There are other reasons to be careful and get legal advice if you have any reason to worry. They have said that they cannot exempt you from the "trafficking" provisions that cover the tools to circumvent things.

So it's pretty easy to hit a Catch-22 where you're "allowed" to do the act itself, but it's difficult or impossible to legally acquire the tools needed.