I'm genuinely curious about this question. I think the answer is "very", but I'm interested in other opinions.
Geo-targetting is a system whereby different content is delivered to users depending on their geographical location. The concept isn't new. Consider the more progressive Pizza joints in town. They have a single number for the entire city (or country for all I know). When I dial it from north east Calgary the phone rings in the closest location to my house. If someone else dials it from south west Calgary, they get the location nearest them. Another example is the continent-wide 1-800-PICK-UPS phone number for the UPS courier service. I get the Canadian office when I dial it, but an American will get the US office.
That's pretty simple stuff because the telephone system switches transmit ID data that gives up the location of the caller. The same principal applies to the Internet, but it's the IP address of the user that signals their location.
Why do I think it's important to know where listeners are coming from? Well, if podvertising is going to become attractive to local advertisers, then there has to be some way to ensure that the advertiser is only paying to have their ad put in front of relevant listeners. If the local Sears is having a Labour Day sale at all Calgary locations, it's important that those ads are only served to listeners that are within a reasonable distance of Calgary.
The big innovators that are experimenting with podvertising are doing so with international products. Go Daddy and GoTo Meeting are big supporters but they can afford to be because it doesn't matter where in the world listeners are from. Everyone can register a domain on Go Daddy and anyone with a Windows PC can use GoTo Meeting.
Kiptronic has reportedly been working on offering Geo-targetted ads which is where we have to go. Ads need to be inserted on the fly and they have to take the geography of the listener into account. If I download Podcast X from Calgary, I will hear the Sear's Labour Day sale ads. If someone from Ontario downloads it, they will either get no ads, or ads that are relevant to them.