In the universal sense it is not necessary for a man to be celibate to be ordained a priest. It is, however, a discipline of the Roman Rite for a priest to be celibate.
Ordinarily, married men are not permitted to take orders under the Roman Rite if they are married. In no rite is a priest allowed to marry once they have been ordained. Just as for a married deacon, if a priest's wife dies he is not permitted to remarry.
Even in the Roman Rite there are exceptions to this otherwise mandatory discipline. these exceptions are primarily the result of a pastoral provision authorized by the Venerable John Paul II to allow men who had been Anglican(Episcopal) priests before converting to Roman Catholicism to be ordained. This was done for many reasons, one of which was, as might be expected from the title of the provision was pastoral. In some cases these men did not enter the Church alone, but brought along their congregations. JPII allowed these individuals to continued to minister to these flocks, after a suitable period of training and guidance. In fact he created a whole new version of the Roman Rite for them known as the Anglican Use.
Since that time other Anglicans have been accepted into the Church, and because of their experience have been allowed to serve as priest even though previously married., even serving in ordinary parishes, though seldom or never as pastors.
It is likely that we will see a small influx of such men in the near future. Benedict XVI has authorized the creation of a brand new structural entity, the Ordinate, under which parishes, and even diocese, of Anglicans may (re-)join the Catholic Church. Like military diocese these Ordinate will span diocese, their parishes under the pastoral control of their Ordinary, rather than the local bishop. The Ordinary many be a bishop, or he may be priest, abet one who will have some of the prerogatives of a bishop. Initially many of the priest of these parishes, and even the Ordinaries will be drawn from those who have up until now been married Anglican priests.
Such men will have to be in valid marriages, that is they can not be divorced, unless their previous marriage has been validly annulled. They will also have to be ordained, since their Anglican orders are not valid. But they will be priests of the Roman Rite, though they might celebrate according to the Anglican Use, or not. Such details are still being worked out.
They will undergo discernment, so the validity of their call to the priesthood will be determined as carefully as any other candidate. They will be part of a very small group, priests of the Roman Rite not called to the discipline of celibacy. They do not represent a change of heart on the Church's part. Merely a pastoral approach to true Christian unity, so that we may all be one..