i know there is a variable, function, or stored procedure that you can use to find the path that SQL Server is installed to:
e.g.:
c:\Program Files\Microsoft SQL Server\MSSQL.7\MSSQL
or
m:\CustomInstance\MSSQL
In reality, i'm hoping to SELECT for the default backup path. But since i doubt that exists, i'll just tack \BACKUP onto the install path and call it close enough.
Update One
select filename from sysaltfiles
where name = db_name()
Server: Msg 208, Level 16, State 1, Line 1
Invalid object name 'sysaltfiles'.
select filename from master.dbo.sysaltfiles
where name = db_name()
filename
----------------
(0 row(s) affected)