Continuing the recent theme of replying to oneself....
http://ubuntuforums.org/showthread.php?t=977332
The php CLI binary is expecting input from STDIN, and so: php -q myscript.php </dev/null &
.. will work. Seems to be a bug (or mis-feature) in the Ubuntu binaries.
Bizzarely, after all my (failed) attempts at crafting a good search query to find this, I found it by googling for the exact subject of my email.... (Doh!)