Update May-2011 - a little more up-to-date info on using ADF with EBS here.
Whenever I finish showing an ADF 11g demo to people who are working on enterprise applications like Oracle E-Business Suite or Peoplesoft or Siebel - the first question I get is "can we use ADF with our existing application".
Basically they would like to build new UIs on top of their existing systems - and leverage the cool new Ajax based functionality in ADF Faces.
The short answer is YES.
There are two ways of doing this.
One is just build an ADF application directly accessing the database underlying your application.
All you need is the DB connection info and an understanding of the underlying tables.
However one thing to note here is that Oracle EBS and other applications usually use roles when they are accessing the DB from UIs - and using the above approach you are not using the roles/security that are set in the applications.
(update 2010 - you can now use the EBS Datasource to do this - here is a bit more info on the JAAS for EBS solution)
So the more "correct" way of accessing these enterprise systems would be to use SOA - or basically access the applications through the official interfaces they expose. These would basically be web services that allow you to interact with the system using correct roles and security.
A very basic way of doing this from ADF is to use the Web Service Data Control - or just create a java class that access the Web Service and then expose it as a data control.
You can also of course bring in the full Oracle SOA Suite with its set of adapters into the picture.
Some other questions you might have are:
Is this recommended by Apps?
Yes it is one of the top ways they tell you to prepare for Fusion apps. See the "Highlight" section of this page.
Do you have customers doing this?
The answer is yes - we have several customers doing it (for example see Viewsonic story) and we also have Oracle products doing this (Siebel Self Service 8.1). Also see this document for more examples.
Where do I get more info?
Choose the link that match your application:
EBS - http://www.oracle.com/technetwork/middleware/index-098944.html
PSFT - http://www.oracle.com/technetwork/middleware/index-101014.html
Siebel - http://www.oracle.com/technetwork/middleware/index-095206.html
JD Edwards - http://www.oracle.com/technetwork/middleware/index-088030.html