Since I work for a big company that makes various software, starting from free GPL/Open Source systems to many million $ solutions, I could give you an advice.
When you work with GPL software license, or whatever the license it is all about the rights. It does not involve any action. I mean, you have to give the right to your customer to use your source code, although you don’t need to FORCE them to use it. By default, everything you develop with most of the scripting languages is deployed to the customer’s system as source code, since it is not compiled. If you give the copy of your solution to the customer, you will give them all PHP files and SQL scripts, and you obviously give all the source. So, no issues here. You will place GPL license somewhere there, and that’s it. Nobody will read it anyway, and they’ll just press "I agree" like everybody does. Normally, you sell your services to the people that are not programmers and they do not care about those things. Programmers will not buy it from you, they will download MODx and customize it themselves.
So, as long as you provide your code the the customer (which is normally the case, anyway) you are GPL compliant and you will not have the trouble from the client side.
GPL is actually protecting them against you! This is why you don’t have to worry about that.
You would run into problems only if you do something like this (e.g.) - I buy the complete solution from you for my company. I pay $xyz. You give me the system including all the PHP files, SQL, and license. After a year or so, I decide to change a few of your and/or MODx original files (not likely to happen, because I would ask for your support, because you have built the site). At the end, you freak out because of that and you start a legal issue, i.e. you put me on court. Than, you loose

!!! Because I have a GPL ;-)
You got it? It’s all OK for everybody unless YOU YOURSELF want to make a problem.
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What you were saying about hosting a Web site at your machines, it does not concern any GPL issues, because you sell the service of HOSTING in that case, and it has nothing to do with the software. In that case you are the end user of MODx. All the others are using your solution on temporary basis - if they don’t pay for the hosting they will loose the Web site too. Otherwise, you will sell the solution for hosting on the other provider’s HW, but then you will deliver the source code too. I mean - you don’t need to give them the FTP access to the source code.
Think about this other way - have you ever paid for the hosting to another provider? Probably yes... They had facilities such as Apache, PHP, MySQL, Linux, etc. How many times were you provided with full source code of e.g. Fedora Core 5, MySQL 4.1.7, etc by your provider?
So, you know... That’s hosting - your clients RENT a solution from you and you don’t have any obligations with software licenses there.
I hope this helps
Igor Obradovic