John McCain was assured the 2008 Republican nomination because on Super Tuesday he won "winner take all" states of New York, New Jersey, Connecticut, and California. His lead was so big that Mitt Romney quit.
Things will be a little different in 2012. The 4 early primaries (Iowa, New Hampshire, South Carolina, Nevada) will in February and Super Tuesday on March 6 may include Arizona, Michigan, Florida, Ohio, Georgia, Missouri, Virginia, Tennessee, and Texas. That's a formidable group of states. If they were winner take all in 2012, a candidate winning several key states (e.g. Texas, Florida, Ohio) would have it all but wrapped up. The delegates will be divvied out proportionally at any March primary. It seems likely that at least two candidates will still be vying for the nomination. The next major primary will likely be Illinois on March 20, but the rules make this state also divide their delegates proportionally.
The first primaries that could be, but don't have to be, winner take all are Wisconsin and Maryland on April 3. Neither are big enough to end things.
There looks like a break of 3 weeks after that before New York, Pennsylvania, and Connecticut are on April 24. If a candidate takes all the delegates in New York and Pennsylvania, he or she may have enough delegates to get the nomination.