Georg Zimmerman in Winboard Forum have asked for utility, which will truncate
a PGN file that each game should be maximum X plies long. Here you are. 
This is only a first approach, so it may have bugs. Please report them to me. 

Known limitations/shortcuts:

a)game is truncated after comment or illegal move;
b)no long filenames support;
c)no footer result.

Usage:

LGPGNTR <InputFile> <OutputFile> <max.plies>

Enjoy,
George Lyapko
