Trusting someone else to print you out wallet private keys is just insane - even if the OP of this thread is trustworthy, it sets a dangerous precedent allowing any asshole scammer to come along and repeat the trick.
I have added a means to verify that my sheets are genuine, I'm stamping them with my own fingerprint. See
http://casascius.com for a reference fingerprint.
By doing so, at least I have raised the bar upon any asshole scammer. I at least have a real life identity to put on the line!... something scammers are rarely willing to offer.
but now, anyone else can include your fingerprint on their own sheets.
since trust isn't needed for a product in this niche, it's unfortunate to require it. for example, you might just package and sell an open-source version of grondilu's script (or write your own simple front-end to openssl) connected to some small additional code that generates a pdf from the output. on a system with the right tools, it's a ten-line shell script. of course, your customers might copy it to their friends without a license from you, but that risk seems easier to price than the risk that you'd retain clients' keys forever and, some time far from now, simply steal bitcoins untraceably.
alternatively, given the nature of ecdsa keys, you could let customers construct something printable on a website while provably not having access to their private keys. see gavin's 'split private keys' discussion in the development forum, from which you can extrapolate what you need. (being out of practice with the mathematical rather than the systems-related features of cryptosystems, i did not initially realise myself how easy it was to do this, but it's quite straightforward.)