How much information can an IP address provide? Could it be used to
identify an individual?

Dear gareth981, 

IP addresses are composed of four different sets of numbers - which
are actually binaris "translating" into regular numbers. These four
numbers could be used in different ways to identify a particular
network and a host on that network, sometimes, this information could
reach to a certain computer, in others to a certain port, and in
others, to a group of computers using the same network (for example,
in a university or a workplace).

A reverse lookup could give you information regarding the contact
details of the IP address, in case it is registered. An IP address
that is used in an intranet, within an organisation, and not to access
the Internet, would not be revealed with a reverse IP-Address lookup.

Google Lookup: 
Sam Spade is an anti-spam site, in which you could search for the name
and the source of an IP that is harasing you. Of course, in this case
I actually typed a most useful, and very non-spammish, IP-Address :
http://www.samspade.org/t/whois?a=NET-64-68-80-0-1&server=whois.arin.net

As you could see, contact details for this IP address are given in the
site. If you are harassed, you should contact the source's network
providers, if you could find them: some cranks and spammers mask their
identity by forging someone else's IP address. You could read more
about it in my answer here:
http://answers.google.com/answers/main?cmd=threadview&id=50182
"Tracking down someone".

I hope I helped you and answered your question. If you need any
further assitance or clarification, please let me know. I'd be pleased
to clarify my answer before you rate it.

Informative