solafide,
Since you addressed glenster about what I wrote, I might just as well reply. :)
You misunderstood my point. Of course "Trinitarianism is monotheistic" -- unless you have a very (too?) narrow definition of monotheism. And this is the point: "monotheism" is not monolithic. It comes (and historically came) in many forms and shades. Among the factors of difference within monotheism is the "inclusive/exclusive" parameter. It plays on many levels: for instance you can construe "God" as exclusive or inclusive of "gods". There you have the theology of Deuteronomy (which started as monolatry rather than monotheism: one god exacting exclusive worship from his people but not denying the existence of other gods, hence having grounds to be "jealous" if his people worshiped other gods) vs. that of the patriarchal stories of Genesis for instance (where everybody, including the Pharaoh or the Philistines, seems to worship the same God under different names). But you can construe him as exclusive or inclusive of "creation" as well. One side borders with late Gnosticism where the Father has nothing to do with creation. The other borders on pantheism where "God is everything".
Both Trinitarianism and Arianism are 4th-century doctrines. You cannot assume either of them to represent exactly late 1st or early 2nd century thinking as reflected in the NT. A lot of things changed in between, especially in the aftermath of the 2nd-century Gnostic crisis. Early catholic "orthodoxy" defined itself against Gnosticism in many ways, but it also inherited a lot from Gnosticism -- in particular the notion of an absolute separation between "God" and "creation". The orthodox "God" could claim "creation" as his (rather than the messed up work of a fallen demiurge) only inasmuch as he was considered totally separate from and transcendent to it. This was a new criterion of exclusivity, which both 4th-century Arians and Trinitarians shared as a common presupposition. Hence the big issue became which side of the (new) demarcation line the Son was: created or uncreated? A question that would have made no sense to the NT writers, whence the ambivalent "answer" their texts offer when asked this anachronistic question.
The Trinity doctrine is, in a sense, more inclusive than the Arian one, because it includes the multiple within the one God. But it is more exclusive than Johannine theology, in that it excludes the believers from "God". To get approximately the scope of Johannine theology in orthodox doctrine, you have to add different "chapters" to "theology" stricto sensu (= "doctrine of God"): christology (incarnation and hypostatic union), pneumatology ("God" indwelling the believers as "Spirit"), and eschatology inasmuch as it allows for final deification of the believers (an important theme in Eastern orthodoxy, rather toned down in the West though).
About John 14:9, I didn't mean it cannot be understood in the Trinitarian framework. It certainly can, but through a great deal of conceptual complexity (especially the Cappadocian notion of perichoresis) which painfully compensates for the loss of a more inclusive theo/christology as that which is found in GJohn in a much simpler way.