I am a wedding photographer and take up to 600 photos at each wedding.
 I currently run, or BATCH, an ACTION in Photoshop CS that allows me
to automatically take each one of those 600 photos and resize them
into a 4x6 proof that I can send to the lab for printing, then give to
my client.

I am looking for an ACTION that will do everything above, plus allow
me to WATERMARK each of those files (vert. or horz.) with the FILENAME
of each image.

Here is the software link and how to do it.

BreezeBrowser
http://www.breezesys.com/downloads.htm

You can do this by setting the watermark text to @file@. You can also
use most of the other tokens available to the HTML generator in the
watermark text e.g. @date@ for the date the picture was taken, @model@
for the camera model.

My filenames setup by Downloader Pro goes like this:

slug_062605_H4567_rx.jpg and
slug_062605_C6789_rx.jpg when using two different cameras during a shoot.

If I use @fileAsNumber2@ to watermark the photo, then all I get is
4567 or 6789. How can I get the full *five* characters to show?
Wouldn't @model@ be too longwinded a string?

You could get DLPro to store the five characters in the one of the
IPTC fields (e.g. the object name) and then use @IPTC_objectName@ to
extract this in the watermark text.