If the first full moon that follows the March equinox falls on a Sunday, then Easter is postponed until the following Sunday. That is the case in 2011. The full moon will occur on 17 April, but since that is a Sunday, Easter will be celebrated the next Sunday. That is what makes Easter come very late this year, as late as I can ever recall it occurring.
The reason for this goes back to the fourth century. At that time, there was a dispute about when to celebrate the Lord's Supper. Some maintained that the only proper date was 14 Nisan which corresponded to the first full moon after the March equinox. Others said that while the Lord's Supper was indeed important, the Resurrection was even more so, and since Christ rose from the dead on the Sunday which followed that full moon, they preferred to observe that. To keep their Easter celebration separate from that of the Quartodecimans, or the "Fourteenthers" who insisted on the 14 Nisan observance, steps were taken to make sure the two celebrations were separate.
Passover figures into this as well. There is almost always a full moon for Passover since it falls on "the fourteenth day" of the lunar month of Nisan. A lunar month always begins with the new moon, making the fourteenth day the one on which the full moon will occur. Very often the Witness observance of their Memorial will correspond to Passover's date, or will come a day or so either side of it. That is not the case this year with the Memorial falling on the 17th and Passover beginning on the 19th of April.
The Memorial will almost always come before Easter. Very rarely will the two fall on the same day. That is also the case for Easter and Passover. In 2012 the first full moon after the March equinox occurs on Friday, 6 April. Passover will be observed that day after sundown. Easter will come on Sunday, 8 April. Jehovah's Witnesses will likely observe the Memorial that Friday, but there is also a chance the Governing Body will choose Saturday, 7 April for the "celebration". I don't know if next year's date has already been determined.
Quendi