Well, I know Kovid's take on this (
see his signature); it should be "calibre", but why do I feel so inclined to give it proper noun status, as in "Calibre"?
Even the MobilRead.com Calibre forums are on "my side"
as you can see by the description at the top of this page i.e. "
Calibre New! Calibre is an open-source..."
What do you think? Does it matter?
(In the end it only matters to one person...
)