Option 1:
The warranty will start on date of purchase of the serial number is registered through the system, like at an apple store when they scan the UPC and then the SN.
Option 2:
The warranty will start the day you turn it on and set up Lion/ML. The activation portion of the set up will activate the SN.
Option 3:
If you buy an open box item, if the item is repurchased within 60 days of the original purchase/open date the retailer can reset the warranty to your date of purchase, this can only be done by the retailer and needs to be done within the 60 day window.
Option 4:
The Mac is sitting sealed in inventory for an extended period of time. The warranty will AUTOMATICALLY start after a set period of time from the date of assembly, it's usually a year. So for example if you buy a sealed Mac from a store right now that has Lion on it, since we're in the days of ML, the warranty will have already started. To solve this, call AppleCare and have them reset it.
How do I know this? I work retail and sell Mac products every day.