To locate someone by an IP Address requires a court order be served to the user's Internet Service Provider.
Most everyone on the internet has a Dynamic IP Address. This means that a broadband user may have a particular IP today, but within 24hrs someone else will have that IP. For dial-up users, their IP changes each time they dial in to their service provider.
Generally, tracing an IP Address only gives you the name of the internet service provider and basic location (Country, State, City) for that IP Address. However, in rare instances a user may have a Static IP Address which can be traced back to the owner of that Address Block.
The whole "IP Tracing" issue becomes pointless as most users today are discovering proxies, both public and private.
PROXY: "A proxy hides your IP Address by stripping it from all data packet headers, adding it's own IP then forwarding the packet. On the return trip (back to the user) the proxy reverses the procedure."
Using this procedure, "an IP trace may reveal Singapore as the country of origin, but the user may actually be in the U.S. or even right next door."
TIP:
Avoid using the same username at multiple sites such as newsgroups, forums, myspace, blog postings or message boards.
Avoid postings using your email address or other personal information, such as including address numbers, street or city names in usernames and passwords.
To see the amount of personal information that we freely reveal about ourselves and understand the impact of those revelations, simply conduct an internet search for your own username.