Amazon Elastic Compute Cloud (Amazon EC2) is part of Amazon Web Services. It provides "resizable compute capacity in the cloud"---in other words, it allows you to run what are essentially virtual private servers of various sizes and capabilities. It is relatively straightforward to set up a WebObjects application server on an EC2 instance. There are several flavours of Linux available as base images to customize.
Amazon EC2 presentation from WOWODC 2009
You can get more information about Amazon EC2 and WOLastic in the presentation Ubermind made about it last year at WOWODC West 2009.
Public WO AMIs
An easy way to get started is to fire up an instance of one of the public Amazon Machine Images (AMIs) that are already configured as WebObjects application servers.