Bah. I've been happily using Google services with a non-Google email for years now, but when I created a gmail account for that ID, all my notifications from all google products (e.g. google calendar notifications, notifications from other Google products like Google Code) suddenly started going to the gmail mailbox instead of the address they'd been going to all along.

Fortunately I was able to delete the gmail account to reverse this, but it was relatively difficult to find the "turn off gmail" button. And if all new accounts get gmail, they may stop letting you turn off gmail at all.