This compiles allowing concurrent modifications to AA:
---
shared string[string] t;
void main() {
    t["a"] = "b";
}
---

Though it doesn't look like AA is thread-safe.
And functions like
T rehash(T : shared Value[Key], Value, Key)(T aa)
can lead to believe it is.

It should be also documented whether AA is thread-safe http://dlang.org/hash-map.html because there's no legible AA source to consult.