Do you believe it would be legal for me to write a software
application that made use of the database originally created by
another?  In this situation all customers that would purchase the new
software have already purchased this existing application, that has
created data in Oracle and SQLServer accounts that the customers are
responsible for administrating themselves.  Is there such a thing as
copyright of a database schema?  If I were to export the data from
this database to another, would this be different to accessing the
original?  The application is in use in the UK and the US.

Just for info, the schema is nothing amazing, it'd just make upgrading
to our replacement system more painless for these customers if there
was as little changing as possible.

The background is that the US company we work for now is selling its
product to a middle-eastern software house.  Amazingly, some of us
don't want to go along for the ride!  We were originally put in our
own offer to buy the product, but the price they're asking is
ridiculous.  Got around to thinking that as most of the software is
decrepit anyway, we don't actually need it if we could get the
customers to switch to a brand new one (totally different code and
graphical presentation).

License agreements generally will disallow this activity, as it would
constitute what they consider altering the functionality of the
program.  Whether or not that agreement is enforceable or not is a
different question.  Have you checked the license agreement?

The recent 7th Circuit case called AT v. Wiredata located at
http://www.ca7.uscourts.gov/op3.fwx?submit1=showop&caseno=03-2061.PDF
(you have to download the file and give it a .pdf file extension to
open it) is right on point.  It holds that you cannot use copyright
law to deny access to uncopyrightable facts.  Current copyright law
holds that most databases are merely compilations of raw data, and raw
data is not protectible in copyright.  Now, the trick is determining
whether the underlying database really is nothing more that an
assemblage or raw data or if the authors have imbued it with enough
creativity and originality in the selection, arrangement and
presentation of that data such as to make those aspects of it
copyrightable.  See Feist Publications, Inc. v. Rural Telephone
Service Co., 499 U.S. 340 (1991)
(http://caselaw.lp.findlaw.com/scripts/getcase.pl?navby=case&court=us&vol=499&page=340).
 See also Mywebgrocer v. Hometown Info, (2d Cir. 2004, No. 03-7909,
July 13, 2004) (http://caselaw.findlaw.com/data2/circs/2nd/037909p.pdf)
(holding that even an otherwise uncopyrightable assemblage of facts
may merit copyright protection if evidence suggests some creativity
was employed in the selection, order or arrangement of the otherwise
unprotectible facts.)  Because of the uncertainty in the case law and
because of the fact current statutory law does not protect true, raw
databases, there have also been recent legislative efforts to protect
databases, despite what copyright law says.  You might want to check
on the status of H.R. 3872
(http://thomas.loc.gov/cgi-bin/bdquery/z?d108:HR03872:) and H.R. 3261
(http://thomas.loc.gov/cgi-bin/query/D?c108:2:./temp/~c1080b7KRe::)

So, if you write an application that pulls data from another
preexisting database, you are not infringing US copyright law unless
it is determined that that underlying database does merit copyright
(or statutory, if those laws pass) protection.  That is a very
fact-based inquiry, and I think you need to consult with a copyright
lawyer to whom you can show the actual database.

If I can write an Addin for MS Excel and sell it, I don't see why you
can't write an "add on" for this program and sell it...but that's just
my humble opinion.