This could be done with an event sink although it would initially be loaded
into the SharePoint database  and then moved to the file system.

But why would you want to do this? Microsoft's direction is to move
everything into the database. In the Windows Longhorn version of the OS, the
file system as we know it will be replaced entirely with SQL Server.

Try "My Network Places". To the users they think they are browsing
files, but it is pulling their lists from SP.

I know some people in my company were complaining that if they had to
attach a file from a doc lib to an email, they had to download it,
then attach it. I then showed them to click on "My Network Places" in
the left pane when they attach a file and they can browse right to any
document library  and attach the file that way.

I *think* you can even mount a document  library as a drive letter so
essentially, the files in SQL look and act like they are on a file
system. Just throwing out suggestions.