My Mac is in the database, but it returns the start date of coverage beginning more than four months before I purchased it from Amazon, so I question whether larger resellers that move lots of hardware send such information to Apple. The serial # isn't recorded on the invoice like I've seen small resellers do.
The date listed is about a month after it was manufactured, so it probably corresponds to the date the machine was sold or shipped to Amazon.
There are links on the Apple site to submit a missing serial #, or to correct a purchase date, but they require a proof-of-purchase to be faxed to Apple. Who still uses fax machines? I certainly don't have ready access to one.
I suppose I could scan it and e-fax it, or find some other way, but if I do need to invoke coverage, I'll always have my hardcopy receipt to show them. The only trouble I see is trying to get phone support since it thinks I'm outside the 90 day coverage period, but I would hope that the situation would be easy to explain/argue over the phone.