[jQuery] Selection broken in Firefox 3.5.7

Hi,

I have the following code to ensure at least one report checkbox is
checked on my page

if ($("input[type='checkbox'][checked]").size() == 0)
{
                  $("#error").css("color", "red").text("Please select
at least one report.");
                  return false;
              }
              return true;

This worked fine in Firefox until I updated to Firefox 3.5.7.  Now size
() always returns zero even if I have a checkbox selected.  Why does
this no longer work?  Is there a better way to write this code?  Note
that I do not have easy access to the IDs of the checkbox controls
because they are dynamically created with ASP.Net.

Thanks.