One of the best ways to record an interview with good sound quality, where the guests are at two different locations, is to do a double-ender. No, a double-ender is not something you are going to hear about on shows such as Open Source Sex. A double-ender is when both parties each record their end of the conversation and then the two pieces are brought together, synced up, and then edited. Great care needs to be taken to make sure that both ends are indeed matched up correctly.
One trick to match the files is for one person to place the headphones over the mic and then have the person on the other end do a quick countdown. This way, his or her voice is recorded on your end and their end at the same time. Then in post production you get two tracks that are very easy to match up.
This method is used by many podcasts where the hosts are in two locations. The biggest advantage of this method is the great sound quality you get when you go this route. You can both be talking via cell phones and it will not matter, because all people will hear at the end is the recording through each mic directly into the computer. You get audio quality equal to having the guest in your studio, but you and your guest are relaxed because both are in familiar surroundings.
There are some major and minor reasons double-enders are not used more in interviews. On the technical side, you need to make sure you sync up both files correctly, and you need to get the guest's end of the recording sent to you, which requires at least a modicum of computer savvy. It also requires a broadband Internet connection because an interview recorded to an uncompressed audio file is sure to generate a very large file.
To send very large files (up to 1GB), use yousendit (www.yousendit.com). This free service allows you to upload your files to the server. The service then sends a link to the email address you supply, for the recipient, to download the file.
Another problem with doing double-enders is that you can pretty much forget about this method if you are going to be interviewing a lot of people, especially if any of the guests are not podcasters. There was zero chance that Rob was going to ask Phil Gordon, Larry Kudlow, Walt Mossberg, or Senator John Edwards to record their end of the conversation and then spend half an hour uploading it to the Net for him. It all comes back to respect of your guest's time, and doing a double-ender takes additional time and effort on your guest's part that can be avoided using the recording method mentioned earlier in the Skype section.