It seems that old-school "dual classing" is mostly covered in the retraining rules, so I don't really know what they're hiding in their bag of tricks for whatever this is going to be next. I'd be interested in a gestalt thing, but 4e kind of struggles with the idea of niche protection + no accidental suck. These two concepts together means that, first of all, no Defender can be as good at being a Striker as a Striker is, but that also, if they're expected to be a Striker, they can't be as good a Striker as they are a Defender.
To put it another way, you can never be as good a Rogue as you are a Fighter (you're a Defender not a Striker), but you also can't just be someone who isn't that great of a Rogue or a Fighter (so you can't be a Defender/Striker and just be a "lower level" defender/striker).
I'm pretty confident that you'll NEVER see a "true" multiclass build where someone does both classes with full effectiveness. The closest you'll get is probably "hybrid" classes like the Swordmage who is a fighter who can also use a lot of controller-esque powers (but who still isn't a full fighter/wizard).
Given that, I'm interested in seeing what's coming down the pipe for that. I'm already a bit of a fan of the "non-classed" multiclass feats presented in the Art of the Kill article, so I think their strategy has merit, even if it won't please those who want to do two things with equal skill.