Matt Jones points to Matt Locke's thoughts on public caches, "standalone devices embedded into street furniture or trains that people could use to upload or download content." The key feature -- each device would be standalone. This would reduce the likelihood of one content provider spamming the network (they'd have to physically travel to each node), and create interesting effects related to "slowness."
Interestingly, we came up with no examples where slowness itself was an asset, but a few of the effects of slowness were interesting. One was privacy/security - the Public Cache network most closely resembled the physical networks of drop-boxes and other intelligence/spy techniques. The process of uploading/downloading should be anonymous, like the messages left under park benches or empty trees in many a cold-war spy story.
This is pretty much the reverse scenario of something I envisioned about a year ago; an opportunity for Sony to leverage their wifi-enabled portable servers into a content distribution network localized to high-traffic locales like airports, train stations, etc.
If I had $500 to spare, I'd plop a wifi-enabled drive on my roof, stick a high gain antenna on it and open it up as wide as possible, just to see if folks in our (reasonably high-traffic, tech savvy, Starbucks-network equipped) neighborhood would (a) notice it and/or (b) do something with it.