In Mahayana Buddhism, what is the relation between nirvana and phenomenon? I have seen nirvana described as the unborn, unconditioned, etc, but then how is it possible to experience nirvana if the experiencer is a conditioned phenomena? I feel as though there is a causal connection implied by that, which would make nirvana dependently originated and subject to permanence.
Since you said individual interpretations are ok, here is mine. I'm approaching this from a secular Buddhist perspective, one which treats Buddhism as a psychology and rejects anything unscientific.

Everything is conditioned, including Nirvana.  Furthermore, since enlightenment is realizing the self is conditioned, it makes no sense to claim the enlightened "self" is unconditioned. 

However, one can use "unconditioned" in a relative sense when speaking of Nirvana.  Our contentment is usually a function of what we encounter, and in this we are conditioned.  Yet if our contentment is relatively independent of what we encounter (Nirvana) then we can speak of us/Nirvana being unconditioned -- relatively speaking. 

Again, this is to be taken in a very relative, phenomenological sense.  Our biology is still conditioned, what brought us to Nirvana was conditioned, etc... Yet our experience feels unconditioned as our well-being is not dependent upon the usual conditions, and in that sense we can say we are unconditioned or that Nirvana is unconditioned.

I hope this helps.