could 'hosting' troubles be PHP 4 vs 5 issue?

Add me to the list of customers whose (current) sites are completely down this morning. ...going on hours now.

One thing I've noticed, though, is that an older site I have ON THE SAME SERVER as my newer sites runs just fine, while the newer ones are completely inaccessible.

The only difference between the sites that I can think of: the older one was set up to use PHP 4, while the newer ones all use PHP 5.

Anyone else seeing this?

I don't think so, I have PHP4 and PHP5 regardless all my sites are inaccessible as of this morning, (and the saga continues)

Anyone have any thoughts on why the one older site ON THE SAME SERVER as my newer sites would work just fine, but the newer sites won't even come up?

I believe all new sites are now running on apache 2 whereas older sites run on apache 1.3. It could be that your machine's apache 2 is broken but the apache 1.3 is ok. Check the response headers on your sites (when the new ones are working again) and compare. Or check the announcement/newsletter archives and see when the change was made and whether that timeline makes sense considering when you set up your sites.