iPhone 3G Map Management

I want to use my GPS map on the iPhone to help keep track of business contacts. Is there a way to enter large quantities of locations and store them on the phone so that I can see which customers are close to where I am? Because I travel outside of the country, I would prefer to have it stored on the phone, sinse data usage outside the USA can get unreasonably pricy.

Preferably, I'd like to be able to turn that function off when I am not doing work stuff.

Can I use the regular map function for that or is there an app that does this?

Thanks for any suggestions...