Well in my head it's "Sirra", but I guess "Skirra" is fair enough if you want to pronounce it like that It doesn't matter, nobody talks about us in 'real life', heh.
Originally we were going to be "Scirrus", as in the Cirrus cloud with the Sci- prefix (it's the latin root for knowledge, or something). Scirrus.com was already taken, so a tweak later 'Scirra' was born.