Look at mx.utils.UIDUtil
But (from doc) :
"Generates a UID (unique identifier) based on ActionScript's
pseudo-random number generator and the current time.
The UID has the form "XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX"
where X is a hexadecimal digit (0-9, A-F).
This UID will not be truly globally unique; but it is the
best we can do without player support for UID generation."