I have no doubt the technology will within a few decades exist to connect multiple minds together directly. Brain-computer interfaces have been researched since the 70s (DARPA) and significant progress has been made.
But anyway, would we still call them "dreams" if minds were communicating with each other? It depends on the method and execution, I suppose, but I would be tempted to consider it more a sort of shared virtual reality experience. When in a dreaming state, the body is essentially paralyzed while the mind remains active in an "unconscious" state. We can effect the paralysis with technology and "disconnect" the mind from sensory inputs (auditory, visual, etc.) so as to keep the "dreamer" unaware of reality, but could truly unconscious minds communicate, or would they have to be in [some sort of] a conscious state?
Of course, even if the subject were not really dreaming but were in fact conscious, he could be tricked into thinking he was dreaming. Projecting a reality that his conscious mind will assume
must be a dream would probably not be hard, and I imagine the use of sedatives would help in more ways than one.
As for the "planting an idea" part, Inception is vague about just how "deeply" implanted it must be. Like others have said, planting ideas in our heads without us knowing it already happens. I think the depth with which an idea can be planted will depend not so much on technology as on an understanding of neuroscience and the individual's psychology.
Ironically, I feel like I'm in a half-conscious state right now and about to go to bed. Been up 40 hours so forgive me if this is nonsensical.