HTML5 supports the attribute multiple="true" which enables a file inpit field to have multiple files uploaded at once.
The files can be fetched on server side just as if they were send by multiple file input fields with the same name[].
An example can be found here:
http://www.3site.eu/jstests/upload/
The feature is known to work in Safari 4 and the Chrome 3 - I'm not sure but FF 3.5 could be in too.