Typically, the XCode installer is on one of the system install DVD you got with the MBP. It is not part of the normal system install but in an Optional Installs directory. You can install that and then use Software Update to bring it up to the latest version. You can do all that without signing up for a developer account, free or otherwise.
Sooner or later Apple will release XCode 4 to the public. At that point you may need a developer account to get it. Software Update will likely not upgrade you from XCode 3 to 4.