We do indeed use id and a few other custom attributes in our OPML. We'll be documenting them soon, not to create a standard, but to instruct developers what they mean, so they can also build 'stuff' ;)
The thing is, that we *could* use namespaces - but PHP5 can make that a pita. So, I chose to use attributes. Nice and simple. And any OPML parser out there should just ignore them, as the defaults are provided.
Ignore them, until we provide the simple docs that is ;p
