Was the script hosted at millisecond.com or at your own server? This makes a difference and is explained at length in this help topic: http://www.millisecond.com/support/docs/v3/html/articles/websubjectnumbers.htm. Possibly relevant quote:
"Subject number assignment for scripts hosted on other servers
that is responsible for generating the subject number. For
random generation, the
method looks like this:
return (Math.floor(Math.random() * 1000000000));
Note that we are selecting numbers with replacement, so it is theoretically possible that two participants might be assigned the
same subject number. However, the chances are slim indeed, and if it does happen, you can use the time of the session as
logged in the "date" and "time" data columns to distinguish the subjects' data."
Also note that the mere fact that some subjects have the same number does not necessarily skew the distribution of even and odd numbers.
Hope this helps,