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The Hospitaler swaps existing domains for two on the Hospitaler list. The description specifically says that "Access to other domains is lost."
I haven't seen that opinion posted, but I presume it's based on the statement in the PrC description that Hospitalers "seldom go on quests or adventures except to rescue endangered pilgrims or to recover relics or sites from heretics and unbelievers."
Just a point, in the FAQ they removed all that domain non-sense (since there are two conflicting spells descriptions in the book) and replaced it with a simple +1 caster level per hospitaler level. That means no bonus or lost domains.