I made a tiny patch for issue 297 [1]. This fix works this way:
1. When the text (username/password) is copied to the clipboard, it is
also copied to a static string variable in Safe.java.
2. When the safe is locked, the clipboard is compared to this static
variable, and if it is the same as the last stored password, the
clipboard is cleared.
Safe.java isn't terminated until the application terminates, so the
static variable we need is retained.
[1]
http://code.google.com/p/openintents/issues/detail?id=297