The problem is that flock file locking is advisory only. Which means it's only good if everyone "plays by the rules".

If you have a copy of the Camel (3rd Ed) handy, this is discussed in some detail in Chapters 16.2.1 and 23.2.2

I believe that I have read somewhere that it is possible to get around this, but I can't recall the source.

Cheers,
Darren :)