Its a mistake to think Buddhists are passive. Recent happenings in Burma tell us that.
Thais are attracted to Christianity in part because of the warmer social setting which goes along with the Thai Christian scene.
When I attended the Union Language school I met some of the farang preachers, good people imo. I stay in contact with one fellow, and recently told he started preaching in Thai.
The way I see it working here in Thailand is Christianity is presented as an add-on to the existing Buddhist culture not necessarily as a substitute.
Christianity has historically melded with indigenous pagan culture. The roots of Christianity itself are of pagan rituals.
In bkk the church has a pretty big presence, from what I saw they aren't doing any harm.
Also it was interesting to note how the Koreans have taken up Christianity in a big way. There were lots of Koreans at that school.