Hi David,
I've been a user for about a year now and have not encounterd any issues with Marketo up-time (hopefully I'm not jinxing anything by saying this). However, if the systems were down I believe they would notify all Marketo users with a login of the issue and any visitors to your landing pages would be directed to the fallback page you selected in Admin.
If you want to view Marketo's online status, you can view that here: http://www.marketo.com/trust/