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brotherhood_of_LAN - 3:26 am on Apr 9, 2010 (gmt 0)
Did you get this working?
I saved your HTML on a test.php page, on its own, and it worked fine.
Output using print_r($_GET):
Array (
[main_page] => index
[Flt_Grade] => Array ( [0] => 100 )
[filter_id] => 4
)