Saving changes is not permitted. The changes you have made require the following tables to be dropped and re-created. You have either made changes to a table that can’t be re-created or enabled the option Prevent saving changes that require the table to be re-created.
This is different than the default in Sql Server 2005, where you can automatically drop and recreate a table in the designer. Solution is simple, and doesn't require you to lose your changes.
In Sql Server Management Studio 2008, go to tools à options, in the options window, left pane Designers, and on the right side uncheck Prevent saving changes that require table re-creation.