In literal style of scalar type (what you used in example) line brakes needs still to be "escaped" (in this case intended correctly).
And you can only have printable characters.
I am not fammiliar with markdown, but in case you would need to save unprintable characters, you would definitelly to escape them.
From Yaml specification:
To ensure readability, YAML streams
use only the printable subset of the
Unicode character set. The allowed
character range explicitly excludes
the C0 control block #x0-#x1F (except
for TAB #x9, LF #xA, and CR #xD which
are allowed), DEL #x7F, the C1 control
block #x80-#x9F (except for NEL #x85
which is allowed), the surrogate
block #xD800-#xDFFF, #xFFFE, and #xFFFF.
On input, a YAML processor must accept
all Unicode characters except those
explicitly excluded above.
On output, a YAML processor must only
produce acceptable characters. Any
excluded characters must be presented
using escape sequences.