Two more silly ideas for encapsulating IP.
IP-over-IRC, where packets become PRIVMSGs or labelled channel messages between hosts connected to a common IRC server (with plain channel messages for broadcasts, and perhaps CTCP ACTIONs for ARP-like mapping hosts to nicks).
IP-over-digital-audio, where a soundcard's digital IO facilities can be used for slow digital communications. The advantage of this is that many soundcards include fibre ports for digital audio, so this could provide a low-cost fibre point-to-point link.
The Digital Voices project is experimenting with carrying data over audible background noise as a communications mechanism for ubiquitous computing; I particularly like their data-over-cricket-noises demonstration.
Oleg Verych not only had the same idea, but implemented it to provide full Internet access from an ISP that only gave access to IRC; see his ipox project.