As to the OP, The Roman dogma of the Immaculate Conception is contrary to the beliefs of Orthodoxy. If Bishop Kallistos Ware's comment in his book, is meant to say that Orthodox christians can hold as an opinion the roman dogma, then he is wrong- once again(he is quite the lousy Orthodox theologian).
The RC definition of the Immaculate Conception is as follows:
"The doctrine which holds that the blessed Virgin Mary was preserved from all stain of original sin in the first instant of her conception, by a singular grace and privelege of Almighty God, in consideration of the MERITS of Jesus Christ, Savior of the human race, has been revealed by God and must therefore be firmly and constantly be believed by all the faithful."
This definition is full of difficulties for Orthodoxy. First off, we are taught that the Theotokos was concieved by parents who were spotless themselves, because they themselves descended from a lineage of ancestors who were all holy. St John of Damascus writes, "Joachim and Anna, how blessed and spotless a couple! You will be known by the fruit you have born as the Lord says, 'By their fruits you will know them'. The conduct of your life pleased God and was worthy of your Daughter. For by the chaste and holy life you lead together you have fashioned a jewel of virginity..." In the Matins of the Birth of The Theotokos we worship, "Ann, the barren and sterile, was not childless before God: For she was foreordained from many generations to become the mother of the pure Virgin, from whom the Maker of all creation sprang in the form of a servant."
A more damaging blow to the IC belief about the Theotokos being "preserved from all stain of original sin in the first instant of her Conception" is found in the Matinal Canon of the Service of the Annunciation, where a dialogue between the Angel Gabriel and the Theotokos takes place. About being overshadowed by the Holy Spirit the Virgin Mary responds:
"The descent of the Holy Spirit has purified my Soul and sanctified my Body, it has made me a Temple that containes God, a tabernacle divinely adorned, a living Sanctuary and the Pure Mother of Life." This event recalls Like 1.35-38
Another un-orthodox aspect of this dogma is the reference to the "merits" of Jesus Christ. That the crucifixion and ressurection of Christ in 33a.d. actually saved Mary by going backwards thru space and time, thus saving her before it actually happened- is not Orthodox. In fact this belief renders the process of Theosis void. Instead, as the holy Liturgical texts of the Orthodox church teach us, "O pure Virgin, sprung from mortal loins, thine end was conformable to thy nature..."(Dormition of the Theotokos). Christ Alone is the Immaculate Conception and any definition attempting to establish "when" or "how" the Theotokos remained sinless is futile and is a mystery .