I have been stamping my larger hard cheeses on their last turn through the press so that I can keep them straight in the cave later. Plus, this just looks cool when you put out your cheese on a platter at a party:
(http://i186.photobucket.com/albums/x214/Mighty-Jesse/IMG_20111213_172628.jpg)
It's simple and cheap. I got a full set of clear stamp letters (or two) from the scrapbooking section of my local JoAnn Fabric (or Michael's) craft store, and before the final turn through the press, I arrange the letters on the plate under my cheese hoop. You MUST have cheese cloth between the stamp and the curd, or the curd will not knit properly as the whey will not wick. Happy stamping!
What are they made of?
Clear stamps are made out of a photopolymer (thus allowing you to make your own images if you care to.) The websites that make them have the following to offer about their safety: Cured photopolymer stamps carry no health risk (we have not tested for FDA compliance or food compatibility). They contain no latex, phthalates, melamine, carcinogens or toxic or harmful compounds.
It is also interesting to note that photopolymers are used as dental fillings (My sister is an orthodontist.), and are considered bio compatible and resistant to most of the things that humans put in their mouths... I have not had any problems with my stamps in extended exposure to the acid environments caused by expressed whey or in disinfecting them afterward with a bleach solution.
MJ - I would use your stamps on the very last pressing without the cheesecloth. In other words, press as normal, remove the cheesecloth, then press naked using your stamps at a fairly low weight. You will get a much cleaner image.
Sailor - I tried that and the impression didn't stay. I do a lot of medieval cheeses that shrink a good bit after pressing due to exterior salting or brining. Unless the impression is good and deep, the stamp fades. I agree the impression is easier to read with a lower weight pressing but for the varieties I tend toward, the deep impression and cheese cloth to draw whey seem necessary. I've done a fair amount of experimentation with them, but I'm not 100% done yet. ;)