Role db_ddladmin not working properly on SQL Server 2008
EDIT:

I re-read your question and I may have got it incorrect before. Looks like you are running the scripts using SSMS instead of using T-SQL scripts. I verified that it works when using T-SQL scripts and doesn't work when using SSMS GUI. To get around this issue, you may want to use T-SQL scripts.



This is a known issue and looks like this is fixed in DENALI CTP3 and I can confirm that.

Ref: https://connect.microsoft.com/SQLServer/feedback/details/467582/db-ddladmin-role-doesnt-allow-use-of-design-functions-in-ssms

Just verified the (correct) behavior also works in SQL Server 2008 R2. I don't have a SQL Server 2008 instance to verify unfortunately.