This is one of those issues I encounter occasionally and I always have to google. So I'm thinking if I put it here it'll save me a couple of key strokes!
I am often developing databases in SQL Server, currently primarily 2008, including modifying tables through Enterprise Manager. I didn't get this issue in 2005 but the default in 2008 seems to have changed as I get this error:
Saving changes is not permitted. The changes that 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.
So obviously the issue becomes a) finding the option (not easy!) and then b) remembering where it is for next time.
FYI/ FMI in the future, it's here: Tools->Options->Designers->Table and Database Designers->Prevent Saving changes that require table re-creation.
Hope this also helps others! Also you can help me as well - is the default the same in 2012?
28/06/2013 - as I've just installed and hit the same issue, yes it is!