Service Pack 1 for SharePoint 2010 (which can be found here http://www.microsoft.com/download/en/details.aspx?id=26623) add the functionality of being able to restore deleted sites from the recycle bin. This is a great thing that SharePoint has been needing for quite a while but if you don't know it is there or how it functions it can cause you some grief. There are several things to know if you have this in place.
1. Deleting a site simply puts it in the recycle bin
2. Quota space is not freed up until the site has been removed from the recycle bin
3. Database space is not freed up either until it is removed from the recycle bin
4. Removing from the recycle bin is a resource intensive step and the system can become unresponsive for minutes (or longer).
So, my recommendations are you can move a site to the recycle bin at any time but you should remove it from the recycle bin only during off hours. This is especially true for sites that have large amounts (read gigs) of data. This process is hard to stop once it gets going and can cause some serious performance issues.
Hope that helps,
SharePoint Ninja#servicepack #SharePoint #deleting #Sites