How about going the other way around?
if imported():
break
<do your main code>
Most of the time, people put the main stuff at the end of the script, so this check can serve as a seperator, equivalent to what is sometimes marked with '***********************' - snip the script here.
Of course, it is still possible to do
if not imported():
main()
I think it is at least as obvious as is_main(), and even more so for those without a C-like background.
(Personally I'd prefer an `imported` magic variable but I guess that's out of question).
Elazar