If True it would run the scheduled functions due to expire soonest (if any) and then return.
This would make sched module usable into asynchronous applications.

I think this would be better served by a separate method. You could call it e.g. run_nowait().

What about run(nowait=...) or run(only_ready=...)?
Doing this as a separate method seems unnecessarily complicated to me in terms of implementation (move run logic into _run, add "run" and "run_nowait", etc...).
Most importantly, the user will have to remember two methods which are basically equivalent in terms of "what they actually do".

That's a good point. Then perhaps call the flag "wait" or "blocking", since it avoids false positives and is more explicit than "async"?

"blocking" seems the most explicit to me.
With this, we can also fix issue1641 by providing a specific section into asyncore doc which explains how to use asyncore in conjunction with sched.

New changeset 2975618965c0 by Giampaolo Rodola' in branch 'default':
Fix #13449: add 'blocking' parameter to sched.scheduler.run() so that the scheduler can be used in non-blocking applications
http://hg.python.org/cpython/rev/2975618965c0