Amy: I don't believe there is an option to turn this off, but it's a good idea to suggest to Marketo! I think Marketo have recently made an update so that you only need to set an upload alert once and it will save this forever - or at least that's how it appears to function.
Alison: To turn these alerts on, when you're next uploading a list of data, go to the top right corner of the screen where it shows the number of leads processing, and hit the "Enable notifications" link, then enter your email address and save. Be aware though that with Marketo's new fast imports, you only have a window of a few seconds to get this enabled for the current import, but even it's too quick it'll be enabled for all future imports at least :).
Hope that helps - drop me a like if it did!