There is no such function in the new ext/dom..

Any idea where I may find documentation about the difference between old and new dom compared to php4.3 and php5.0?

I'm having the same problem with Redhat 9 and PHP5.0.0-RC1 (snapshot). By pulling up a list of all available functions it is confirmed that the domxml_open_file does not exist.

I know that it will take a while for all the documentation to reflect the major changes in PHP5, but is there a means how we can get the documentation for the DOM XML/XSL extension?

<?
$c = get_declared_classes ();
sort ($c);
print_r($c);
?>

By running the above code you'll get the following list of classes:
--snip--
[13] => domattr
[14] => domcdatasection
[15] => domcharacterdata
[16] => domcomment
[17] => domconfiguration
[18] => domdocument
[19] => domdocumentfragment
[20] => domdocumenttype
[21] => domdomerror
[22] => domelement
[23] => domentity
[24] => domentityreference
[25] => domerrorhandler
[26] => domexception
[27] => domimplementation
[28] => domimplementationlist
[29] => domimplementationsource
[30] => domlocator
[31] => domnamednodemap
[32] => domnamelist
[33] => domnamespacenode
[34] => domnode
[35] => domnodelist
[36] => domnotation
[37] => domprocessinginstruction
[38] => domstring_extend
[39] => domstringlist
[40] => domtext
[41] => domtypeinfo
[42] => domuserdatahandler
[43] => domxpath
--snip--

The indexes will differ, but that is besides the point. The DOM interface can now only be access through these classes.

For documentation on these classes you can try the libxml2 homepage at www.xmlsoft.org, but I'm not sure if the docs correspond to the PHP implementation.

Use the W3C docs for some hints, how to use it.

Or my slides from http://slides.bitflux.ch/phpug2004_1/

Or wait for the Zend Article about that, which should be 
published soon.