Ikiwiki could support building some pages into additionnal output formats, such as postscript or plain text, and provide links to those on the html output.
This would provide true "printable versions" of the wiki pages supporting it.
Could this be done by making the output format a plugin, similar to the way pyblosxom works? Atom and RSS could then possibly be moved into plugins.
Presumably they'd have to work by converting HTML into some other format, as trying to force all input languages to generate more than one output language would be impractical to say the least.