security concerns with cable splitters

[NSweaney at tulsacash.com (Nathan Sweaney)]

I just received a question that I can't answer.  A customer has a cable
internet service with COX that has only been used for internet.  The
modem and all other networking equipment is locked away so that no one
has access to it.  They've decided that they'd also like to have a TV in
a public area for visitors to watch.  COX says to just add a splitter in
front of the modem & run a cable to where they want it.  The crazy thing
is that the customer actually considerd the security implications and
asked ahead of time.  
 
So my question is, if an intruder had uninhibited access to the coax
that was split off upstream from the cable modem, is there anything they
can do with it?  I've been told by COX that it won't interfere with the
connection and that adding a second modem to the connection wouldn't
work because it wouldn't be setup on their end.  However my bigger
concern is the potential to intercept traffic.  
 
I know from past experiences that if you plug your coax into your VCR or
satellite the wrong way you can actually deliver content to your
neighbors (or at least cause a lot of interference), so that suggests
that the connection doesn't just flow one-way.  So can the
coax-splitters determine which way traffic is supposed to be flowing?  
 
I understand that even if it's "possible" it may not be easy or likely,
but this network is used for processing credit cards so I want to make
sure I have a complete answer.
 
Any information or resources on this would be appreciated.
 
Thanks
 
- Nathan Sweaney
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