As luck would have it if you forget to add your own name SQL Server will add one for you. with a very easy to remember name. If DF__Beta_foo__bar__5629CD9C is easy to remember that is. So all of that being said, here is a quick script to get a list of all the constraints and the tables / columns they are on so you can easily add them to your rollback script.
SELECT b.name AS TABLE_NAME, d.name AS COLUMN_NAME, a.name AS CONSTRAINT_NAME, c.text AS DEFAULT_VALUE FROM sys.sysobjects a INNER JOIN ( SELECT name, id FROM sys.sysobjects WHERE xtype = 'U' ) b ON ( a.parent_obj = b.id ) INNER JOIN sys.syscomments c ON ( a.id = c.id ) INNER JOIN sys.syscolumns d ON ( d.cdefault = a.id ) WHERE a.xtype = 'D' ORDER BY b.name, a.name