My profiles:
People who use the Amazon.com web services routinely complain about their sluggishness. In the dedicated discussion board, after someone suggested they create an XML feed to advertise the current availability and average response time, as well as planned downtime, a developer from Amazon said they'd look into it. Better late than never, I advocated something similar for Paypal in late 2001.
Sometimes I wonder whether their web services are anything more than a cool gizmo for Amazon.com. They certainly don't give the impression they consider this a mission critical tool. Affiliates who use Amazon's web service need to cache the data and avoid live calls that might break their own site.
10/05/04 update: Rips in the Web 2.0 fabric.
11/05/04 update: Bringing Web Services to the Masses.
02/26/06 update: Trust.Salesforce.com.
09/24/07 update: this is finally something more companies are doing now, e.g. heartbeat.skype.com.
09/01/09 update: Google Apps Status.
I know about this blog, it's a step in the right direction. They should have something machine-readable too, so that your software can act accordingly (e.g. you ask Paypal in real time whether they're available, and if they aren't, you route your customer to an alternate payment gateway instead of leading them to failure and losing some orders).