Can someone find out who I am?

Possibly, but most likely not without a subpoena.

In most cases all that can be easily determined is who the person's ISP is and possibly the state and city the reside in.

In some cases with static IP address DSL or business DSL services the ISP provides IP allocation records to the American Registry for Internet Numbers (http://www.arin.net), the non-profit organization responsible for managing Internet numbering resources in North America.

As an example SBC used to include customer information in those allocation records, but due to privacy concerns now uses their own information with a customer reference number.

For users who want to afford themselves some additional level of anonymity we suggest you read about internet anonymity, Proxy Servers, and anonymizing services.