Well, this explains a lot - first, a serious congratulations to Joshua, and kudos to Yahoo for picking up del.icio.us. (and thanks to Les Orchard for the heads up)
I've been wondering about the business model for del.icio.us since before I started using it - wondering how it was going to be paid for. It's a free service, with no ads. Soaking up ungodly amounts of resources and bandwidth. Now that it's Yet Another Yahoo Family Member, it should benefit from Yahoo's infrastructure, and lose the imperative to "monetize" - since it's part of the value-add for Yahoo.
Here's hoping the first thing the Yahoo server fairies do is throw some über high-end hardware at the service to help alleviate the occasional performance issues...