A few thoughts on coin anonymity using TOR...
TOR is very capable of anonymously routing normal internet traffic (albeit a little slowly) so using the existing wallet options -> proxy setting it should already be possible to hide a client IP address via TOR, the network would see the client as connecting from the TOR exit node's IP address. (I haven't tested this with Dark wallet, but have run various other applications over TOR via normal application proxy settings, and as long the application uses the proxy for all traffic and doesn't 'leak' information via DNS requests, etc then anonymity should be preserved)
Of course this wouldn't provide any anonymity to miners using either home computers to mine to their wallets (mining traffic -> Home IP address - > ISP account -> Name) or mining from cloud accounts (cloud account -> credit card -> name). These miners would also need to use a TOR proxy - this setting exists at least for the CPU miner (and I assume sgMiner/ccMiner too).
Various agencies have already run up against the requirement to easily identify bitcoin wallets and no doubt will be looking at targeted traffic monitoring as a tool to this end.
If we therefore assume the possibility that all crypto coin network traffic is being logged and all related account details can be requested from ISPs, cloud providers, etc, then I think the only way for 100% anonymity would be to setup a clean TOR proxied wallet and mine from behind TOR proxies to fill the wallet. This would result in an untraceable wallet, but provides limited usefulness unless large mining resources are available. Coins obviously could not be purchased at an exchange and no transfers could be received from any other 'unclean' wallets without potentially linking the clean wallet with a third IP address which might be able to identify the clean wallet. ("Mr 3rd Party, tell my friend the rubber hose who owns the wallet to which you transferred x coins last Tuesday")
In my opinion this is why Dark Send is such a differentiating factor, combined with TOR for IP address hiding (for those who will go through the effort to set it up), it potentially provides the missing 'untraceable transfer' part of a truly anonymous payment system.
I do think master node availability as a TOR hidden service with a .onion address and the optional ability to force the wallet to only use these masternodes via TOR would provide a significant additional boost to the overall anonymity of the process and ease of use for the average user. Dark send not withstanding, the less public information provided the more anonymous the system. It would be very interesting to investigate what barriers exist at a protocol level (if any) to prevent interested individuals setting up TOR hidden service master nodes.
Sorry for the rambling post!