OK Mr Monster, here is how I see it now that I know exactly what you're asking:

You can mix and match internal and external IPs in your own network without
breaking anything so long as target machines use external IPs and all routers
on your network are properly configured.  However doing this isn't considered
a best practice.  Furthermore, by doing this, you could be in violation of 
RFC2050 (this is a must read!!) because you could potentially be using external
IP addresses internally  that never get hit by outside clients.  This is
considered a big no-no and as time goes on, ARIN will become more aggressive
with companies that don't comply with RFC2050.  For example, last year Ford
Motor Company lost two class B networks.  They were using these class Bs 
internally and had them firewalled from the rest of the world.  Two years ago
the city of Philidelphia was rejected for a third class B because they weren't
in compliance with 2050.  IBM lost two of their class Bs last year for the same
reason.  

The standard accepted practice is to use all internal addresses for everything.
To hit a target inside your company (which if it isn't proxied, it is a security
risk) you'll probably want your firewall to statically translate it.  

Mr Monster, also I appreciate you making me think.  No one here at work is 
capable of making me do that :-)

	Mike Neuliep