It depends very much on how larget the file are. The real problem of flatfiles is that (unless you use a very space-wasting method) you always have to read everything leading up to the part you want. at 1k files that no problem, at 10M files it is.
Plus flatfiles are single-user, no two people can post at the same time.
In short, forget using flatfiles for forums. Use a database.
A forum, a FAQ, email notification, what else do you need?