Blog search engines and feed reading services can and do take pings from Weblogs to understand when content is updated. But it's not the only service you can ping, if you want to send out notifications.
But why wouldn't you depend on pining Weblogs? Right now, there's no reason not to. But while Verisign pledges pinging will remain free, it also talks about adding "value added services" for a fee. Potentially, that could mean getting some pings out faster than others. Or not. We don't know, and it remains a "we'll see type of thing."
The main point is this. Ultimately, I think pinging will be heading down a path of pinging the most important places in addition to any centralized server. So if Weblogs suddenly did get all into charging for rapid response, you'd still be able to notify other places despite it.
Technorati has its own ping server, and I expect that any major player may ultimately have their own, perhaps with registration barriers or other systems to cutdown on ping spam. Any smart marketer will hit these in addition to a centralized server.
The entire pinging thing got you down/head spinning? Basically, most of this is probably built into the blogging tool you use. But a good starting place for more is Ping-o-Matic, which helps you send pings manually if you need to, plus has useful background.
By the way, in addition to Verisign buying Weblogs.com, the ping system, AOL yesterday bought the entirely different Weblogs Inc. network of blog sites.