In the USA it is legal to receive most unencrypted radio frequencies (those that are blocked by law are always disabled in retail scanning radios sold here).

Some agencies stream their dispatch over the internet for enthusiasts to listen to in other cities or if they don't have a scanner. You can search Google and find the same streams you would in that app.

Natively, these are over-the-air (usually analog) broadcasts similar to the FM radio you can pick-up with the N900 (commercial FM radio uses 87mHz through 108mHz here) but on a different part of spectrum (UHF and VHF).

I have always wondered if the crystal in my US N900 is the same as international versions (anyone?) and therefore capable of receiving more VHF frequencies since the FM radio band is different in each country.