Earlier discussion here: https://bitcointalk.org/index.php?topic=128230.msg1621711#msg1621711 and https://bitmessage.org/forum/index.php?topic=4170.0

It all boils down to: Using the same key for signing and encryption seems to work in theory but might cause security issues in practical use. At the very least it is even more difficult to get right so better avoid it.