jQuery.fn.filter also works with callbacks (apart selectors, and elements).
jQuery.filter doesn't.

I would propose to integrate jQuery.grep inside jQuery.filter and to make those aliases.

Also consider that there is intention to add a jQuery.forEach that would be inline to the ES5 spec, and making jQuery.filter inline with that spec too would be a logical move anyway.

Here is a my try: http://gist.github.com/290689

If this would happen, jQuery.filter will be moved from traversing.js to core.js, I hope.