Oh I'm sure Google keeps those logs for a long long long time, like most companies who aren't deliberately trying to "protect your privacy and anonymity". In fact isn't there a law now about it?
If nothing else, it's fodder for future types of data mining, ad focusing, search engine tweaks, etc.
AMDMaddness: It's not strange it was the same IP address. If I send an email through my ISP, it'll almost always be the same IP address because they don't change them very often (dynamically assigned addresses, but they usually just renew with the same one). Only a dial-up user is likely to have a different IP each time they connect. Also, remember that the email is NOT being sent from that person's own computer, as it would be with an email client like Outlook. It is being sent from Google's own webmail server, the "client" is running on that machine, so the originating IP will always be that server's IP (or the IP of a local server if they use different ones for each region).
Is your gmail username just your last name? Even if somebody else had the same name as your daughter, why would they just happen to send an email to your email address thinking it was her father?
I think replying to the message was the very last thing you should have done. It may have turned out well this time (although it's still not definite that it has), it could have just fed whatever creep was doing it and prompted further actions.