Most printers keep a copy of what they print in memory. And some printers do so for a very long time (up to 2 years in my experience, it had an internal hard drive). So be sure to destroy your printer before getting rid of it if you print your GPG key with it.<p>Also, many new printers comes with internet connection (some even over 3G) and contact the vendor when ink is running low or maintenance is needed. The problem that they are not always well protected against attacks from the network.<p>At my university, we had a case of a printer which has been hacked and many giga of scanned and printed documents were downloaded. Fortunately the attacker couldn't find a way from the printer to other devices on the network (computers, hubs…), but some people had scanned their ID card for instance, which was a problem they really hadn't forseen…
Hm, random idea: print your key on paper and have a unique fold pattern that "decrypts" it. It'd be obvious how you folded it if you keep it around past a one-time thing, but you could print a dozen of them and keep them non-folded.