25 March 2012 falls on a Sunday. The Solemnity of the Annunciation is consequently celebrated on the following day, 26 March, just as it was in 2007. This is in accordance with the General Norms for the Liturgical Year and the Calendar, §5:
Because of its special importance, the Sunday celebration gives way only to solemnities or feasts of the Lord. The Sundays of the seasons of Advent, Lent, and Easter, however, take precedence over all solemnities and feasts of the Lord. Solemnities occurring on these Sundays are transferred to the following Monday except in the case of their occurrence on Passion Sunday (Palm Sunday) or on Easter Sunday.
You will find a number of web sites that move the Annunciation to 24 March instead. This is because the Norms were revised relatively recently, and the earlier version of the Norms moved the Annunciation to the previous Saturday instead of the following Monday. Many liturgical books date from before then, and many web sites, including some diocesan ones, reproduce the text of the Norms in their earlier version.
For those who want the Latin text (as included in the Third Edition edition of the Roman Missal, published in 2002), here it is:
Propter suum peculiare momentum, dominica suam cedit celebrationem solummodo sollemnitatibus necnon festis Domini; dominicae vero Adventus, Quadragesimae et Paschae super omnia festa Domini et super omnes sollemnitates praecedentiam habent. Sollemnitates autem in his dominicis occurrentes ad feriam secundam sequentem transferuntur, nisi agatur de occurrentia in Dominica in Palmis aut in Dominica Resurrectionis Domini.
The only exception to these rules is in the case of the Solemnity of St Joseph. The first edition of the Norms moves it to the day before; the second edition moves it to the day after except that if it is not a holy day of obligation, the bishops may move it anywhere outside Lent; the third edition moves it to the day after unless it falls on Palm Sunday itself, in which case it is moved to the day before, but if it is not a holy day of obligation, the bishops may move it anywhere outside Lent.