While the Cloud Print designers look forward to a future where the typical printer is cloud aware, they realise that's not going to happen quickly. A method is therefore required to use existing printers.
The proposed approach is to build a print proxy that will be installed as part of Google Chrome (the browser, not the OS). This proxy will implement the communications functions of a cloud-aware printer, and forward the job to a locally attached or networked printer.
Work is underway on a Windows proxy, with Mac and Linux versions to follow.
The downside is that the computer needs to be switched on. But that's not a fatal flaw - the proxy is being put forward as a way of making existing printers work in this environment, not as a primary implementation of Cloud Print. Do you leave your USB-connected printer switched on when the computer is powered down?
In any case, there's nothing to prevent the design and sale of what Google calls a "proxy in a box" - an Internet connected device that provides the equivalent functionality.
Cloud Print could save a lot of effort - see page 3.