NOTE :
It\'s sort of hard to google for this, because \"GET\" is bringing up a lot of \"how to \'get\' the value of a form with javascript,\" etc.<
Works for me. Just tried
http://www.nearby.org.uk/tmp/multi-test.html
Results in a URL ?multiselect%5B%5D=2&multiselect%5B%5D=3
which will make $_GET['multiselect'] an array in PHP.
Perhaps you have something else in your system, stripping the [] ?