They aren't ignored, but they won't change the post's score (or the reputation of the poster), not even after reaching the required reputation for upvoting. It works the same way as downvoting with less than 125 reputation; your votes are stored as 'feedback' but it is not entirely clear (at first sight) what's done with it. It turns out that this feedback is visible in the moderator tools available to 10k users, and in the Stack Exchange Data Explorer.
After you reach 15 reputation, you need to upvote the post (again) and then it will affect the score.
Update: The message has been changed to make it clearer that you're not actually casting a vote if you don't have enough reputation to do so.