Are you adding a new file to an existing file group, or are you adding a new file group with two files?
If you are simply adding a new file to an existing file group, then no the data won't be moved to the new file. If you are moving to a new file group, then yes.
If you are adding a new file to an existing file group, you'll need to move the table out of the file group then back in, in order to spread the data accross the two files evenly.
Free space from a single file can be reclaimed via the DBCC SHRINKFILE command.
Denny
MCSA (2003) / MCDBA (SQL 2000)
MCTS (SQL 2005 / Microsoft Windows SharePoint Services 3.0: Configuration / Microsoft Office SharePoint Server 2007: Configuration)
MCITP Database Administrator (SQL 2005) / Database Developer (SQL 2005)
--Anything is possible. All it takes is a little research. (Me)