Funny that this topic came up. Before I started training in aikido, I trained relatively seriously in hapkido for 8 years, along with a trivial smattering of other arts. When I watched my current aikido instructor for the first time, I was quite taken with the beauty of the movement. Like Asim, I repeated the mantra of "forget what you know" when practicing aikido, and it seemed to help my progress.
However, what you've learned in the past more or less stays with you...I'm at a point right now where, right or wrong, I'm starting to question what my instructor is teaching me; there are some things he's teaching me that I feel do not jibe with my previous experience in terms of "what works." Perhaps students who come into aikido from other arts will, at some point, feel an urge to put their aikido instructor to the test; I honestly find myself wishing that my instructor will say to me: "Just bring it...come at me with whatever you've got." There's a lecture on the hazards of ego in here; give it to me if you must.
Seeing aikido through the filter of past martial arts experience may, at some point in time, turn you into a doubting Thomas with regards to the aikido you're being taught (in contrast to aikido as a whole, of course). How that gets resolved is between the doubting Thomas and the aikido instructor in question.