Do PlayStation Plus cloud saves automatically update?
Yes, they do automatically update. This feature was introduced in firmware update 3.70 which is super cool :)
Source
and one or two more, just to make sure.
Yes. The playstation blog has a breakdown of the firmware release that included cloud auto saves.
http://blog.us.playstation.com/2011/08/09/ps3-system-software-update-v3-70/