Some new research out today in an article "Preti che 'abbandonano', preti che 'ritornano'" ("Priests who leave; priests who return") by Gian Paolo Salvini, SJ in the Jesuit publication La Civiltá Cattolica (2007 II 148-155, quaderno 3764). The full article itself is not available online but Agence France Presse has released a good summary of the contents in French.
Salvini says the Vatican puts the number of priests who have left the priesthood to get married at fewer than 57,000. This is considerably lower than the 80,000 to 100,000 figure usually cited by married priests' associations. He says that while there are no precise statistics, the Vatican estimates that 69,063 men left the priesthood between 1964 and 2004 and that 11,213 returned between 1970 and 2004. Reasons for leaving were varied and included crises of faith, conflicts with superiors and/or church doctrine as well as other factors in addition to marriage so, while the priests may subsequently have married, it was not always their primary reason for leaving the priesthood.
Since 1967, 438 married priests have asked to be reinstated and 220 have been. A further 114 requests are in process, according to the article. In order to be reinstated, the priest must no longer be married sacramentally, e.g. his wife has died, he was never married in the Church in the first place, or the religious marriage was annulled. He must also be free of any civil obligations towards his wife or children.
Or he could be Eastern rite, ex-Lutheran, or ex-Anglican....