I'm looking for a company that sells residential keyless door systems
that you can change the password over the internet.

I would like to install the system on vacation homes so nobody has to
meet the renter with keys.  The idea would be that i can change the
password for any of the homes from my desk in my office.

I researched this for maybe half an hour, but didn't find much.  Is
this helpful at all?
http://www.internethomealliance.com/pilots_projects/family/onstar_at_home/faq.asp

This sounds like an interesting idea, but when renting places, cars or
boats, I have always felt that the presence of the person giving me
the keys was also to ensure that there were no grounds for complaints
by me about the status at the start of the rental period, thus
protecting the interests of both parties.
It wouldn't be the case with your vacation homes, but elsewhere, the
new occupant  could later claim that damage done by him had been done
by the previous occupant, or vice versa, the landlord could claim that
damages by a previous occupant had been caused by the new one.