The problem of the poetic I and the problem of the poetic subject might seem identical. I would argue, though, that they are closely related but not quite the same thing. Here is where a very technical, rigorous distinction in literary theory becomes relative.
We all know that the speaker of the poem is not the author, per se. The absence of a 1st person speaker, or the separation between this speaker and the author, though, does not mean that the poem does not express the subjectivity of the author on some level. I could express myself through a dramatic monologue, in which the speaker is someone quite different from me, or through a seemingly impersonal painting of a landscape in which there is no 1st-person pronoun.
By the same token, the poetic I itself can be very strange and stylized. The word I means something different in a poem than it does in other forms of discourse, as Carlos Piera has argued, because of the strange ways that deixis works in the lyric.
The you is even stranger: in many cases the you cannot receive the message of the poem (elegy / apostrophe). It is more normal, in fact, for the you not to be physically present at all in the scene of enunciation.