One of the new things that happens in SQL Server 2008 is that it prevents saving table structure changes that require the table to be dropped and re-created. While this is a great feature to prevent accidents from occurring, on a developer machine it can be quite frustrating. This is the dialog you get when trying to make changes in a table design.
Unfortunately, this dialog doesn’t tell you where to turn this feature off! Clicking on the small “?” on the title bar does get you to a help page that tells you how to do it.
Anyway, the place to do it is Tools > Options > Designers > Table and Database Designers > Prevent saving changes that require table re-creation. Turn this option off and you will be able to save the tables again.