I've just uploaded callerid 0.3 to extras-devel, with some new features.

* Now able to show addresses, if you use <address>.
* Added numbering plan database for North America. If anyone in USA or Canada calls you, you'll know in what city they live. (Nobody has yet provided a directory config file for North America, though, so you still won't know their name and street address...)
* Now tries to find the phone number in the N900's own addressbook first. If it's there, don't bother showing the caller's identity, since the phone's own call dialog would be good enough. However, if there's a note in the contact, show that instead (suggested by ejasmudar)

I've edited my original post to explain how to add address and numbering plan support for your country.