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Can I just jumped in here and just say my piece.
I am an IT Manager and in my position, actually encourage my vendors to supply open source solutions to me.
1. I know it has a lower overhead
2. I know it can be supported by ourselves if we need to.
On your questions on GPL - well that has been answered. IMHO you can "sell your service" to deliver an enhanced product, packaged under your own product name, provided you do not remove the credits from the source code. Not everyone like to clutter their website with logo of PHP, Apache, Linux, mySQL, XOOPS, etc. As you can see at my website, I dedicated one page to explain the technologies and the Open Source contributors to the site.
On the subject of ethics - your only problem as I see it is with your competitors, not your clients. Therefore I believe it is beneficial to tell your customers you are using Open Source, therefore enabling you to deliver a lower cost software, which is what I (as a customer) wants. And if your competitor decide to deliver Open Source solution, then it is a matter of your excellent service and expertise that will distinguish you from all the other fly by night Open Sources consultants.
I trust that will give you the assurance to proceed with your plans?
I am in principle prefer to see credits to XOOPS displayed on at least one page.