I attended the MIF (Multiple Interfaces) BOF today at the IETF meeting in San Francisco. One hour was definitively too short to discuss this, but there was a strong consensus that more work should be done on this subject at the IETF.
When a host (as defined by the IETF - a network node that does not route packets) is connected to the Internet by multiple interfaces, there is a set of unique challenges to solve. There is more and more hosts with multiple interfaces - a cellphone with Wifi and 3G connection, or a desktop with broadband and a VPN falls in this category. Choosing the right connection to use for a specific destination can be difficult.
What is the most interesting, in my opinion, is that MIF can solve some problems that are basically impossible to solve with one interface. One example is E911, a protocol based on SIP to make emergency calls. With one interface an emergency call using the Internet is less reliable than a 911 call made over the PSTN. There is the obvious reason that the DSL/Cable modem, the eventual NAT and the VoIP device or computer need to be powered, at the difference of a POTS. This can be solved by connecting all this network elements on a UPS but even in this case there is still the problem that the Internet is designed as an unreliable network. There is nothing that can be done about this - this is engraved in the Internet DNA. Your ISP will probably try in the future to sell you a premium connection that "guarantee" you a reliable connection but the bottom line is that a reliable Internet connection is an oxymoron. This is addition to the fact that the acronym ISP should really stand for Intermittent Service Provider, not Internet Service Provider (yes Comcast, I am talking about you - one hour of downtime per month for maintenance between 2am and 3am is not OK).
If the availability of your Internet connection is 99.9% (one hour of downtime per month), then having a second Internet connection with the same availability will boost your global Internet availability to 99.9999 (2 seconds of downtime per month). Obviously you need to choose Internet connections that use very different mediums - if the two connections share the same wire or router somewhere between your home and the PSAP then the availability will not be as good, so one DSL connection and one Cable connection should be a good, albeit expensive choices. Perhaps there is a business model for a company that buys wholesale connections to ISP and resales bundle like this to end users.