IOS – What devices can receive an iOS AirPlay audio stream

Yes.

Airplay is really a technology platform that allows for streaming media on a local network. It isn't concerned with the type of device that is producing the media or consuming it.

To start with, support may be limited and may not come from Apple. However, certainly that will change with demand. For example:

• There is an iOS app called AirPlayer which allows streaming of media to an iPhone/iPod/iPad.
• There is a Mac proof-of-concept app also called AirPlayer which allows streaming of media to a Mac.

Didn't intend to answer my own question, but I found this March 2011 Bloomberg article:

Apple Inc. (AAPL) is weighing an expansion of its AirPlay audio service to include streaming video from an iPhone or iPad to television sets, according to two people familiar with the project.

Under the plan, Apple would license its AirPlay software to consumer-electronics makers that could use it in devices for streaming movies, TV shows and other video content, said the people, who asked to remain anonymous because the plans haven’t been made public. Apple now only licenses AirPlay for streaming audio. Devices that could be used for video may be available this year, one of the people said.

So apparently the answer is "not yet, but maybe in the future."