determining ID-ness in XML

At 20:49 2001 10 28 -0800, Tim Bray wrote:
>The problem is that at the moment, given some arbitrary XML,
>there is *no* good way to determine what's an ID without recourse 
>to some external resource like a DTD or schema, and that, to use 
>a technical term, sucks.
>
> I at 
>least will be arguing in favor of this whenever I get a chance.

I'm interested in this problem too.

Here are some options (all discussed before):

1.  use the internal subset to declare IDs
2.  use a PI to declare IDs
3.  put an xml:id attribute into the xml namespace

Are there others?  What are the pros and cons?

In short, what's the right answer?

paul