if I open up a terminal session and create a new file vitestfile, as soon as I save it I can spotlight search it and it shows up in the Document group of Spotlight's results with vitestfile as the name of the document. If the search happens to turn up file ~/Library/Mail/.../52813.emlx then spotlight shows the results in the Messages group with the email's subject as the name of the document. I suspect the only thing the plugins do is allow spotlight to return a more meaningful description for the results - most people wouldn't know that 52813.emlx was an email message and thus whether or not it was what they were looking for.

What's probably up on the iPhone/iPad is that Spotlight's been changed to not search everything since the filesystem is locked down as far as the end user is concerned. Before the iPad there wasn't much content created on the iPhone, so Apple probably didn't have a way for the third party apps to tell the iPhone's Spotlight engine "index this directory, it contains the user's documents." I wouldn't be surprised in the least if the next iPhone OS update includes an API call to indicate searchable folders.