Catholic News reports that under Pope John Paul II, the procedure was changed to a series of ballots punctuated with time for reflection and prayer if a pontiff was not chosen within the first three days.
In yesterday's document, Benedict XVI restores the traditional norm concerning the majority required for the valid election of the Supreme Pontiff to two-thirds of the cardinals present.
In 1996, John Paul II changed the standard in the Apostolic Constitution Universi Dominici gregis. Under John Paul II's rules, a new Pope was validly elected by two thirds during the first three days of voting.
However, after three days of voting without an election, there would be a day dedicated to reflection and prayer, without voting. Thereafter, voting would resume for seven additional ballots, another pause for reflection, another seven ballots, another pause and yet another seven ballots.
After which an absolute majority was to decide how to proceed, either for a vote by absolute majority or with balloting between two candidates.
This was to happen only in the event that the cardinals arrived at the 33rd or 34th ballot without a positive result.
Holy Father changes rules for electing new pope (Catholic News, 26/6/07)
Pope urges professors to find solutions to 'crisis of modernity' (Catholic News, 26/6/07)
LINKS (not necessarily endorsed by Church Resources)
Papal conclave (Wikipedia)
Anonymous cardinal breaks conclave secrecy (CathNews, 26/9/05)
NZ Cardinal denies conclave lobbying (27/2/05)
Benedict XVI Notes 3 Issues That Need Study (Zenit, 26/6/07)