Actually, in Google, maximum display length has been about 63-64 characters for quite a while. Google's recent SERP redesign, though, makes even shorter titles the default display, and the title element is now effectively about 55 characters, not 70.
About a week ago, I brought up some considerations of this new length in a thread originally titled "Google SERPs larger font effectively shortens title display"
. For clarity, I've changed that to... Google SERP redesign cuts visible title to 55 characters http://www.webmasterworld.com/google/4656190.htm
Note that Google has historically looked at words in titles that it hasn't displayed, but I do agree with deeper's observation that "the user is all", so we need to pay attention to what the user sees.
Additionally, questions of title focus, page focus, usage in title and on the page, concepts vs keywords, type of site, etc, are all going to enter into this. We're probably already seeing changes in how Google deals with title rewriting. I myself would not go longer to pull in extra keywords. But see the thread I mention for questions about things like product page titles, legacy titles, etc. It's not completely clear cut.