I do not use Mail.app, instead using Postbox 2 for all my email needs. I do use iCal for my calendaring, syncing it with my Google Apps accounts.
Whenever iCal wants to send a reminder, it tries to open Mail.app to do so, which is highly annoying.
Currently the only solution I have working is to zip up Mail.app in
/Applications so iCal can no longer access it. However whenever the OS updates (i.e. from Snow Leopard to Lion) it replaces Mail.app and I have to do this again. This seems overly hackish.
Is there a way to force iCal to stop trying to open Mail.app without having to manually remove Mail.app?