Without TLS and having control of the network, it doesn't matter how many channels over the network you use; it's simple to MITM everything and search-and-replace all text matching the key with your forged key (in fact, many networks already MITM all non-TLS HTTP traffic through a "transparent proxy").
With TLS, even with the imperfect CA model, it's much harder. It might have been "demonstrated broken", but can you get a certificate for "fedoraproject.org"? It's not that easy. Add to that the Certificate Patrol extension, which warns the user quite noisily when a certificate is signed by a different CA (and shows the user the old and new CA).
With mailing the CDs, as suggested several posts upthread, it also gets harder; now the attacker has to MITM two things (the network and intercept the physical disks). If you add TLS, it gets even harder (three things: MITM the network, intercept the physical disks, and obtain a valid forged certificate).
So, trusting the CAs is better than getting the key via multiple unencrypted channels through the same network. Trusting the CAs plus getting the key via multiple channels is even better. The methods are not exclusive, and "multiple channels" is already common in practice (in my Fedora example, the DVD image is obtained via bittorrent, while the key is obtained via TLS, and they have to match).