With today's increased Flash storage sizes, there may not be many times when you need to split files. However, when you do need to do so, I find that tar is a good tool (this hint is also good if you want to split an archive before burning to DVDs).
The following use of bash's brace expansion makes creating multi-volume tar files easy. To create a multi-file archive of a given length, use this:
tar --tape-length=102400 -cMv --file=tar_archive.{tar,tar-{2..100}} [files to tar]
To extract from the archive:
tar -xMv --file=tar_archive.{tar,tar-{2..100}} [files to extract]
To create DVD-sized volumes, use --tape-length=4588544. In the above examples, replace [files to tar] and [files to extract] with the location(s) of the file(s) you wish to segment.
[robg adds: I haven't tested this one.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20090321124207437