If you want to see the power of <podcast:guid> we just changed @boobury's feed url from wavlake to MSP and everything switched to the new value blocks. No need to change any <podcast:valueTimeSplit>'s that reference his feed in any other feeds.
Having a universal ID that follows the *podcast* rather than the feed url is a huge deal. The iTunes ID was all we had before, but it's proprietary. Now we have an open solution.