I’d never heard of a lettuce knife before today and this Lifehacker DIY. Google lead me this Wise Geek article. The claim is that metal knives cause browning. How, exactly? Some kind of galvanic reaction? Or is this just marketing BS? This time of year, I’m always keeping an eye out for potential stocking stuffers but I’m skeptical about this one.
The explanation that I’ve been taught a sharp metal knife cuts through individual cells or other small-scale structures in the lettuce. This releases more enzymes and exposes more “sensitive” things to oxidation. A lettuce knife is designed more to tear than to cut (at the small scale anyway) so that these small structures aren’t cut open and don’t oxidize as much. The analogy used is that it’s like tearing bubble wrap (which rarely bursts any of the bubbles because the weaker areas are between them) as opposed to cutting it (which cuts right through bubbles).
Ah, so a dull (metallic) kitchen knife would be just as good? Then we’re golden! 
Probably, yes. I’ve never seen anything convincing to say that it’s the metal causing the problem rather than the nature of the cuts. I’m willing to be corrected, but when I don’t break lettuce by hand (like when I want it shredded) I haven’t sen a difference between my lettuce knife and a relatively dull, serrated bread knife, but I have occasionally noticed the problem with the sharper chef’s knives.
(For what it’s worth, I’ve never seen the problem appear until a day or two after cutting, at the very earliest. If you’re eating the lettuce the same day, I don’t think it matters at all.)
You can also put the cut lettuce into a bucket of water until you’re ready to use it, then use a salad spinner. No browning, and it also washes off any dirt or bugs.
This require empirical experimentation.
Until then, my WAG is that lettuce knives developed out of the notion that clean cut pieces of lettuce are esthetically bad-looking and should be torn, OR, use one of these new fangled serrated plastic knives that leave a rough edge that gives the appearance of being torn. The idea that they prevent browning is a made up etiology.
When I was working in a restaurant that had a great salad bar we were instructed to shred the lettuce by hand rather than cut it with a knife to prevent browning so that any left over could be bagged, refrigerated and used the next day. It seemed to work.
This is what my mother said 50 years ago.
Not sure if the sharpness is a factor, ceramic knifes are sharp and they don’t brown lettuce(or apples etc.) either. I think its to do with chemical reactions.