The effect appears to be small. I don't know if the database logs times of votes, but the times of actual posting are fairly homogeneous, so being a top post early on isn't a big enough advantage to stay a top post forever.

I did notice an interesting trend for the number of upvotes to as a function of place to undulate in an unexpected way, indicating either that people are upvoting slightly differently based on quirks of the number or based on page position in the "top" pages.