[jQuery] number of checked boxes

Hello all,
I have a form with some checkboxes and I would like to submit it
(through jquery) only if at least 1 checkbox is checked, otherwise I
want to display an alert message.
How can I get the number of checked boxes?
I tried $("INPUT[type='checkbox']").is(':checked') but looks
like it doesn't work.
Any ideas about how I can achieve so?
Thanks and have a nice day!
Sig