I'm a preschool administrator.
I chop my waiting list for each year into 3, based on when children are eligible for a funded place. So this year I have a September 2010 sub-list, a January 2011, April 2011 and then September 2011 and so on.
I put children on the relevant sub-list depending on when they are eligible for their funded place. So if your child's 3rd birthday is today, 16th November, they would go on the January 2011 sub-list.
I then sort each of these sub-lists by registration date, i.e. who has been on the waiting list longest.
If the committee decide we can take 30 children, I take the top 10 off each sub-list. This means that we don't disadvantage summer-born children.
Obviously, the drawback is that no.11 on the January sub-list might have been on the waiting list for longer than number 3 or 4 on the April sub-list. And then no, it's not really fair. But as I think Eddas says in her post, there's not really a way that is guaranteed to be fair to everyone. I feel that the way I do it gives everyone a fair crack at it, irrespective of when in the year they turn 3.
But if it's any consolation, in my experience, waiting lists change shape very frequently. Often, people who put their name down a long time ago move, or change their work patterns, or some other thing that means they don't take up their place.