I first encountered this term in Grant Gallicho's dotCommonweal post on the Dublin clergy-abuse report. He quotes the Irish Times as stating:
"Church authorities used the concept of “mental reservation”, which allows senior clergy to mislead people without being guilty, in the church’s eyes, of lying."
This concept is explored in an Irish Times piece Church 'lied without lying'
The New Advent Catholic Encyclopedia entry for this term boggles the mind with its precision and fine distinctions between lying and making statements that are likely to mislead.
DotCommonweal commenter Bill DeHaas offers this reference on mental reservation, its definition and use by the Church.
Well, this scholarly excursion goes a long way towards explaining the archdiocese' press releases regarding the closure of Our Lady of Vilnius. I advise everyone to follow these links. It will help to explain the past and prepare for the future.