Sorry Simon, this is not correct (or at least misleading).

The type conversion happens during the concatenation of the two strings.

-- Example 1:

-- Replicating a non-unicode string results in a non-unicode string

-- (Datalength = Lenght)

SELECT DATALENGTH(REPLICATE('-', 8000)), Len(REPLICATE('-', 8000))

-- Example 2: Concatenating a unicode string and

-- a non-unicode string results in a unicode string

-- (Datalength = 2* Lenght)

SELECT DATALENGTH(N'Unicode' + 'Nonunicode'), LEN(N'Unicode' + 'Nonunicode')