This is a known hoax. If you care to examine the picture and have a keen eye for small details, you can notice that the text has been added later. In reality, there is no such text there.
Beyond the great stuff available, the Internet is full of BS. I am sure that somewhere there is a guy laughing his socks out at how many people he duped.
Sorry to disappoint.
/Admin
I am not so sure, if this picture is a hoax: There is at least one other picture on the web that shows the same text on one of the aft support struts of Shuttle Carrier Aircraft (SCA) N905NA:
http://jetphotos.net/viewphoto.php?id=5790&nseq=6As you can see, the picture was taken from a completely different angle but the text is also there.
So, I am not sure, if this exact writing really existed on at least one of the support struts of N905NA, and if it did, for how long it did or if it still does. The existence of different photos from supposedly different photographers may suggest it actually did, even though this is not "hard evidence", of course.
But: There actually was a similar writing on at least one of those struts for at least some time before 2001, though it was a little different! It read:
CAUTION PLACE ORBI_TER
HERE BLACK SIDE DOWN
LEFTY LOOSY
RIGHTY TIGHTY
A black-and-white picture of it can be found on page 197 of the third edition of Dennis R. Jenkins' great book "Space Shuttle - The History of the National Space Transportation System", subtitled "The First 100 Missions". This picture is credited to Tony Landis, a well reputed aviation photographer, who I believe is credible for not manipulating his pictures.
The caption says "Space humor. Note small writing on N905NA's aft Orbi_ter support strut."
The picture must have been taken sometime before 2001, as the book was first published in April 2001.
(The picture can not be found in the first and second editions of this book.)