I have read the GPL FAQ and done a fair amount of searching, but can't find where this is specifically covered:

Take a non-GPL program, say a music editor for example, that wanted to use a GPL plug-in to encode files into Ogg Vorbis format.

My understanding is that this does not require the main program to be GPL if all of these are true:
- The plug-in is not statically or dynamically linked
- The plug-in works as a stand alone program and is only controlled via command line parameters
- The main program does not package the plug-in as part of a package or install

So far so good? Ok now imagine this:

- The music editor program has a feature called "Encode to Ogg Vorbis"
- When the feature is selected, the program says "Download plug-in to support this feature?"
- If the user says yes, the plug-in is transparently downloaded and run and seems to be a feature of the program, even though it's really standalone with its input and output controlled by the main program.
- On subsequent use, the main program remembers the location of the plug-in and doesn't even need to prompt the user. It now really seems like a feature of the main program.
- The only input to the plug-in are file paths, the only output is the content generated by the plug-in, no internal private data structures are exchanged.

Would this usage require the main program to be GPL?

Any input much appreciated.

Though a lawyer should probably be consulted.. I'd be inclined to think that you could in fact do that, but at the very least I would show the user the GPL license statement that applies to the plugin program and certainly keep a copy of the license with the program. Also, if you are going to provide distribution of the GPL plugin from your website you must also be prepared to provide the source code for said program to anyone who wants it.