From BOL, I see these remarks with respect to the MODIFY FILE subcommand (my underline added):
By default, data and log files are initialized by filling the files with zeros when you perform one of the following operations:
Create a database
Add files to an existing database
Increase the size of an existing file
Restore a database or filegroup
Which leads me to believe that expanding the size of a datafile will also wipe out (my definition of 'initialize') any existing data within that file.
I may be misunderstanding 'initialize', because when I tested it out, I found this wasn't the case - my table data written to the file was still there after a resize.
So I'd be much obliged if anyone can clarify to what degree I'd be taking a risk by increasing the file size on a datafile which already has data in it.