Hi - I'm a DBA also trying to change the default location for new databases, so that my team of developers can create new databases and transaction logfiles on separate hard drives, and I can keep these away from 'master'. I've shown them how to do this, but do they remember?
I agree with Schmo that having to define these locations manually can get pretty annoying. Ideally, I would like system databases (master, model, msdb, tempdb) on Hard Drive 1; new user databases on Hard Drive 2 by default; and associated transaction logs on Hard Drive 3 by default.
There is an entry in the System Registry under 'Microsoft\MSSQLServer\MSSQLServer\Parameters' where the startup parameters for locations of master.mdf, mastlog.ldf, and errorlog.log are specified, but that's all I can see.