Can I assume this has not yet been added (as of 5.03)?
I'm no programmer, but this would not appear to be that difficult of a thing. A subfolder simply lists links to surveys and all this would do is move the title and link to a survey that was originally in one folder to another. I'm sure I don't understand the complexity.
Is it possible to import data into an exported survey? If so, it would also be possible to move a survey in that way. Export the survey (with Reports?) to clone the original survey. Import it from your new subfolder location, then import the data back in? Is this possible?
The reason it has not been done yet, is that there are complexities wrt permissions.
v5.0+ supports export/import with response data, so it is possible to move it this way. Please be aware that the import feature is memory intensive, so it could be difficult with large response counts (1000s). This will soon be optimized.