How to reply to an ics invitiation without using googlemail
It looks like this is a known issue. I'm using K9 mail and, while I would swear I've replied to meeting invites in the past,but I just tried sending my non-gmail IMAP self an invite from my google calendar and I get an ICS attachment and K9 Mail complains that it is "Unable to find a viewer for application/ics"

It looks like there are a couple of apps in the market that will handle this, though. Take a look at iscer and ICS Bot -- I haven't tried either but both profess do to do what you need.