Python: simple async download of url content?
I'm not sure I'm understanding your question, so I'll give multiple partial answers to start with.

If your concern is that web.py is having to download data from somewhere and analyze the results before responding, and you fear the request may time out before the results are ready, you could use ajax to split the work up.  Return immediately with a container page (to hold the results) and a bit of javascript to poll the sever for the results until the client has them all.  Thus the client never waits for the server, though the user still has to wait for the results.
If your concern is tying up the server waiting for the client to get the results, I doubt if that will actually be a problem.  Your networking layers should not require you to wait-on-write
If you are worrying about the server waiting while the client downloads static content from elsewhere, either ajax or clever use of redirects should solve your problem
I have a web.py server that responds to various user requests. One of these requests involves downloading and analyzing a series of web pages. 
Is there a simple way to setup an async / callback based url download mechanism in web.py? Low resource usage is particularly important as each user initiated request could result in download of multiple pages.
The flow would look like:
User request -> web.py -> Download 10 pages in parallel or asynchronously -> Analyze contents, return results
I recognize that Twisted would be a nice way to do this, but I'm already in web.py so I'm particularly interested in something that can fit within web.py .