Intuitively, you would think the IRS would require an affirmative election for a fiduciary to obtain 2011 estate tax treatment for 2010 estates. But the IRS is neither intuitive nor a fool. The Service realized that there would be hundreds of thousands of estates that would want 2011 tax treatment, while only a handful (literally) that would prefer 2010 rules (namely those estates over the $5,000,000 threshold). Putting the affirmative burden on "normal" estates would result in hundreds of thousands of forms for the IRS to process. So the IRS did the prudent thing and made 2011 treatment the default.
Thus, if a fiduciary of a 2010 estate wishes to operate under the 2010 law, with no estate tax and (largely) no basis step-up, he must complete a new form, Form 8939, and file it with the IRS. For the rest of us, the vast majority of estates of decedents who died in 2010 but who wish to operate under the 2011 rules, the fiduciary doesn't need to make an affirmative election on the Form 1041. The IRS will treat those estates by default under 2011 rules.