Myself, I'd place a lot of trust on cperciva, the former FreeBSD security officer, a guy who has written papers on security flaws in Pentium 4 processors. (http://www.daemonology.net/hyperthreading-considered-harmful...)
[edit] paymentiframe.com's FAQ now includes this:
Why should I trust you with my credit card entry form?
You probably shouldn't. This is more of a proof of concept — but you might also want to use the CGI script to generate an iframe which you download (along with the bits it links to) and serve up as a static file from a server you run (but make sure you get a separate domain name for it).