OK, so there are two things here: being able to read the content of the requests and responses, and then being able to modify them. The former may depend on the latter, I'm not sure.
But basically, the question is this: What vantage point would one need in order to carry out an MITM attack? In other words, would an attacker just need to be in range of the unencrypted WLAN over which the target connection is taking place? (Firesheep clearly demonstrates that such a level of access enables one to intercept and read requests, but does it also allow one to modify requests and responses before they reach their destinations?) Would you actually need to have access to the LAN router? Would you need to be an ISP or internet backbone? What level of privileged access is required by an attacker, if any?