This could be very interesting. BlogLines is providing an API (press release; weblog link) that will allow desktop clients – i.e. programs like Jäger – to synchronize multiple copies on different machines; to provide notifications that a blog has been updated; and to provide truncated RSS 2 feeds that only contain new items.

I'm going to size this up this afternoon and decide whether this is worth implementing. I'm noticing a few shortcomings in the API, though some of these comments may be of nitpicking nature:

• There should be a way of marking items as read without re-downloading the feeds
• There should be a way of marking a range of items (possibly just by using two time points) as read – i.e. just because the aggregator has download some items doesn't mean the user has read them on the aggregator.
• Why isn't there a "download everything new" call? The current API requires that each subscription be downloaded as a separate HTTP call; why not have an aggregate function that returns everything in a single call?
• Why isn't there an "upload blogroll" or "subscribe to" API (preferably the later) call? This is really required if desktop aggregators are going to synchronize with each other.
• Why aren't Bloglines' OPML extensions added as a separate XML namespace – i.e. use Bloglines:SubId="3714388" rather that BloglinesSubId="3714388"?

Update: it should also be possible to get the OPML feed with only updated items too.