No, they are not - certainly they are not; at least, Christians are not monotheists in any way which could be explained to an unsympathetic/ hostile non-Christian, e.g. someone who was a straightforward (non-Trinitarian) monotheist.
(Because, if you suppose that hostile critics would be convinced by the one-in-three, three-in-one Athanasian Creed type explanations of why Father Son +/- Holy Ghost are not two or three Gods... then you are mistaken.)
(Since such explanations do not make rational sense, then how and why should anybody who is not already a Christian (of a particularly philosophical mind-set) be convinced by them?)
But then there is no scriptural reason why Christians should be monotheists, since the Bible is full of gods, and silent on the subject of monotheism.
Why then the zealous and punitive obsession with proving (by demanding public assent to statements that - even if they were true - are logically self-contradicting and/ or incomprehensible) that Christians are monotheists?
The reasons for aggressively asserting the monotheism of Christianity are not scriptural, but (presumably) philosophical - and derive from reading scripture through the lens of fixed prior philosophical assumptions: by insisting that God be fitted-into a preshaped monotheistic mould derived from philosophy, rather than by fitting philosophy around what revelation as transmitted by scripture teaches us concerning the nature of God.
Note: I say two or three gods, because I have a hunch that most Trinitarian mainstream Christians do not, in practice, regard the Holy Ghost as a full aspect of the Trinity, that is a personage on a par with God the Father and God the Son - but instead regard Him as the means or mechanism by which the Father and/or Son effect changes among men and on Earth: i.e. more like a physical force than a person. This is promoted by the fact that we do not pray to or on behalf of the Holy Ghost, but ask that the Holy Ghost be sent for our aid and comfort.
Further note added 24 July 2013: The point I wish to emphasize is that the way to deal with the question about whether Christians are monotheists is that it does not matter - not in any fundamental sense.
(Or rather it should not matter and if it does, we are in serious trouble).
Christians are what they are - and what they are should be derived from the proper sources: who cares whether what Christians are does, or does not, fit into some definition or other of monotheist?
Christianity fits some definitions of monotheism, it does not fit other definitions - so what? It is not the job of Christianity to fit definitions of monotheism, or any other philosophical category - this debate is not just irrelevant, but actively harmful to the proper understanding of Christian doctrine.
The primary reality of Christianity is personal and narrative - not conceptual.