From a security perspective - IPv6 SLAAC (sort of like DHCP) does this - it uses the MAC as the last 48 bits of the 128 bit address. However, this is largely seen as a security problem things like RFC 4941 [1] have argued against this idea. If you think having your iPhone keep a track of which cell phone towers you've been to is bad, think about the consequences of being globally reachable by the same IP address at all times, which means you can be tracked to where that devices is and has been based on the routing rules.
As well, ethernet is not IP. They are different layers of the OSI model. Just because you're on ethernet (wired/wireless) doesn't mean you're using IP, and vice versa. Some devices don't have MAC addresses. Frame relay uses DLCI's for instance. Historically there were lots of other technologies using IP that weren't ethernet - FDDI, token ring, ATM, etc. Ethernet has replaced nearly all of those, but not all.
Routing scalability - another huge problem. But this will rear it's head in IPv6 with the exponentially large amount of possible routes. Right now the routing tables are around 350k-400k routes. Compare that to the billions of devices that connect to the internet simultaneously.
MAC addresses are locally assigned addresses. There would be no way to prevent duplicates. In most networks, your IP address will specify the way you are routed through the network and your security level. If that isn't centrally managed you have no security or IP addressing schemes.
It'll certainly be interesting to see if scalability of the IPv6 DFZ routing table is actually better or worse than that of v4 (adjusted for growth in number of multihomed organizations, of course.) One could make an argument that the bloat in the v4 table today is largely due to disaggregated allocations from the RIRs due to exhaustion-avoidance address space allocation policy.
It is entirely possible that large organizations will be able to advertise fewer prefixes, as they have large, contiguous allocations, and thus can more efficiently aggregate at the border.
(Sorry, this is a bit late, but your comment got me to thinking...)
From a security perspective - IPv6 SLAAC (sort of like DHCP) does this - it uses the MAC as the last 48 bits of the 128 bit address. However, this is largely seen as a security problem things like RFC 4941 [1] have argued against this idea. If you think having your iPhone keep a track of which cell phone towers you've been to is bad, think about the consequences of being globally reachable by the same IP address at all times, which means you can be tracked to where that devices is and has been based on the routing rules.
As well, ethernet is not IP. They are different layers of the OSI model. Just because you're on ethernet (wired/wireless) doesn't mean you're using IP, and vice versa. Some devices don't have MAC addresses. Frame relay uses DLCI's for instance. Historically there were lots of other technologies using IP that weren't ethernet - FDDI, token ring, ATM, etc. Ethernet has replaced nearly all of those, but not all.
Routing scalability - another huge problem. But this will rear it's head in IPv6 with the exponentially large amount of possible routes. Right now the routing tables are around 350k-400k routes. Compare that to the billions of devices that connect to the internet simultaneously.
MAC addresses are locally assigned addresses. There would be no way to prevent duplicates. In most networks, your IP address will specify the way you are routed through the network and your security level. If that isn't centrally managed you have no security or IP addressing schemes.
[1] http://tools.ietf.org/html/rfc4941