The mechanisms applied when you "print" can be complex and depend a lot on the printer, the printer drivers and the OS. What you see as text will be translated into something that the printer can accept, and that's the role of the driver on the OS side. In any case, printers don't have infinite memory, so it is the role of some machine (in this case, yours) to handle a "queue" of documents to print, and, guess what, that queue almost always consists in files in a directory. You did not save the document, but it still made it to the hard disk as a queued printable file. In any case, since modern operating systems employ virtual memory, aka "swap space", what you think as "a file which has not been saved" may still have made it to the hard disk, regardless of the printing business.
The printer itself, being network-aware, is also a small computer in its own right, with the same kind of bugs and vulnerabilities. Security holes are made worse in that printer's firmware is almost never updated, so vulnerabilities tend to remain open for long. See this previous question for some discussion on the subject. An hijacked printer will, of course, see everything that is printed.
Though the WiFi connection uses some cryptography, it does not protect against eavesdropping from other connected users; that crypto was designed to prevent unauthorized users from connecting at all, as is made apparent in the acronym "WEP" as "Wired Equivalent Privacy": with a wired network, packets are supposed to be invisible to people who are not plugged in the network, but people who are plugged can still see them. Furthermore, WEP turned out to do a very poor job at its assigned task, but the point here is that the "assigned task" has never been to establish a tunnel between any two systems, safe from all other systems in the network.
Of course, if your own machine is subverted and contains a keylogger, then you have already lost. Your machine is no longer your machine.
Note, also, that the document is not only in the machine RAM and the printer; it also is on your screen, and on the printed paper. Do you have windows in your office (not the operating system, but real apertures in walls, with glass planes) ? That you handle your data with a computer does not mean that classic spying methods don't work any more...