There are already a few dual-SIM phones around on the market, like the Samsung D880 and LG KS660. They are particularly popular in markets for which there are advantages to arbitrage between different carriers' networks, especially for prepay users. Some tariffs do not include "cross-network" minutes, so it's easier for users to simply have multiple SIMs and numbers than waste money on off-net calls.
Most of these phones have been 2G-oriented, although there are some 3G ones emerging as well.
It strikes me that there could be a large market for dual-SIM devices which are optimised to work with one 2G (voice+SMS) SIM, and another for 3G data. So, for example, you could keep your existing voice service while roaming, but benefit from a local cheaper prepaid data SIM.
In fact, even when not travelling, it would be good to be able to disaggregate the voice and data plans, and purchase them from two different operators if that offered better price / coverage / speed. Yes, it would mess up some of the ideas that operators are trying to push with mobile social networks and IMS concepts like RCS, but as an end user I'm not really bothered by that.
Clearly, this wouldn't be for everyone - and there would definitely be value in operators and device vendors offering innovative fully-blended services to try to keep people bundling everything in a single contract. But it would be good to have healthy competition to keep them honest.
Really, the best outcome would be to do away with the legacy SIM card altogether, so you could sign up for the access provider(s) of choice more easily - and switch without hassle. Or perhaps the ownership model for SIMs needs to change, so that you the user (or a neutral 3rd-party) own it, rather than the network.