XML output from Weblogs.com or a Weblogs.com aggregator (like BlogTracker) is an obviously useful feature, and it’s #1 on my to-do list for BlogTracker below all the bug fixes. This would include querying of a particular user’s “filtered” list (gotta come up with a new name for this), which is a form of what you’re thinking.

Including a list of URLs instead of a username in the query for XML only makes sense if you think of the service (Weblogs.com or BlogTracker) as a database that supports raw slice queries. This isn’t very useful considering you could just query the entire feed of aggregated, normalized data and pick what you want out of the results. Might save on bandwidth for the query, but that’s about it. It’s also messy from an API design perspective, because only URLs in the database will return valid results.

It also pushes some of the burden of record normalization onto the data client, which is supposedly part of the value of an aggregator in the first place. If a URL for a site changes, your query would also have to change. With BT-stored user lists, BT has an opportunity to re-key sites in people’s lists to the new URL. BT doesn’t do much of this at the moment beyond basic URL normalization, but it could. (I have some ideas for letting site admins register URL changes, though that’s low on my list right now.)

I suppose BT could expose its internal key for a weblog that it would use to re-track sites when URLs change, but I’m not sure that’s something I want to expose. (Once exposed, I can’t change it without breaking functionality for clients.)

Just a few thoughts.
— Dan

I do exactly the same thing, my own sidebar and my BlogTracker one. I’ll have to steal your code (and we know how *that* goes) and make a system like your new one. 🙂