Make holidays on user level and not tight to a sprint
When entering holidays for a user they are tied to an iteration. It would be better that the holidays are only related to a user and not to an iteration.
You have an iteration starting on 1/1/2012 and ending on 1/2/2012. A team leader Geert registers a holiday on 29/1/2012. If at a later time you decide that you the iteration will only span 2 weeks in case of 4 weeks, you will need to reenter the holidays for the second sprint.
Geert Verhoeven commented
Also it would be nice if we can register half days