We have alerts
in our TODO list. Unfortunately, I am not sure that we will be able to implement this feature for Q2 2009 release, as there are other features with higher priority that we have already planned.
However you could try to implement this using a timer for example that checks every, say every 10 seconds, if an alert should be shown depending on the appointments for which you have set up alerts.
And in the mean time any suggestions for this feature are most welcome.
the Telerik team