On IE 7 and FF 3, the print collate does not work on Vista with either the old or new plugin. I tested this using 6u10 b09 using the following testcase and steps:

http://sqeweb/deployment1/deployment_tiger_exec_ws/deployment/tests/PluginPrinting/pluginhtml-default/PrintDialog_Plugin.html

- Once the applet is loaded, click on the "Native" button. That will bring up a print dialog.  
- Then increase the "Number of copies:" to say 2. 
- Then check the "Collate" box. 
- Now click on "OK" to print.

I printed a few pages with wordpad to verify that the collate does work on the machine and printer that I ranthis tests on and it does.
I am re-dispatching this again, because collate does seem to work on Vista. Try the following:

1) Using WordPad enter some text on the first line
2) Go down several lines and enter some more text
3) Now click on the File > Print Preview. Make sure there are enough lines between the first and second line so that you have at least 2 pages.
4) Now click on File > Print. When the print dialog appears, choose to print 2 copies and check off the "Collate" box.

Notice the pages are collated. Now try running the following applet using JRE 6u12-b01 with IE7:

http://sqeweb/deployment1/deployment_tiger_exec_ws/deployment/tests/PluginPrinting/pluginhtml-default/PrintDialog_Plugin.html

Click on the "Native" button on the applet. When the print dialog appears try to print 2 copies and check off the "Collate" button.  Notice the pages are not collated. I tested this using JRE 6u12-b01 PIT bundle on a Vista 64 with IE7 64 bit.