MSSQL 2008 R2 server roles: do server roles have implicit privileges over *all databases*?

Server Roles are server wide and apply to every DB, the reason you are having problems is because the roles you have granted don't have the correct rights:

• Security Admin: can manage server security.
• DbCreator: can create, alter, drop, and restore databases.

When you say

"I'm running a sample job that should apply some simple amends to a particular database"

I assume this means you are trying to alter an existing database, neither of these roles provide the ability to open or make changes to the content of a Database, in fact the only role that would is the sysadmin role.

So, your options are to either make this group a member of the sysadmin group (which likely has more rights than you want to give), or grant rights on individual databases (either manually or scripted).

The next version of SQL Server will allow the creation of user defined server roles.