My android and blackberry had it but I haven't seen that on an iphone. Unfortunately, company phones require locks/passwords so you cant just leave it unlocked.
It's not too hard to set that up that up on an iPhone. Take a screenshot of a note with what ever text you want on the lockscreen or and set that as your lock screen wallpaper.
Also, for as long as there has been an App Store, there have been apps specifically for letting you overlay message over images specifically for this purpose.