the new flash player (as of 11.4) apparently now supports threads (and concurrency), exposed to the API via the Worker* / MessageChannel family of classes. the approach they took is actually similar to channels in GO, which is nice.
one slight inconvenience is that each time a worker / thread is created it has to exist in a separate SWF (ByteArray). this can be avoided if the actual SWF is created/loaded in/from memory.
a cool new feature, but it might be a little bit delayed in comparison to lets say Java, which became C/C++ like syntax multithreaded / "API exposed", i believe, since 1996 with JDK 1.0.