Multi-channel marketing has been a feature of promotional work for a very long time but it is true to say that its character has changed with the growth of the internet. This was pointed out recently by Chris Rand on his daily blog where he suggests that more people are now involved in decision making than before.
This corresponds to the way that now everybody can become an expert overnight and anyone with the least interest in sales/marketing can participate in decisions about these areas. Each person has their own preferred route to knowledge so it is important for a company to be present across as many as possible. This means that Multi-channel Marketing is even more important now because everybody is a stakeholder in decision-making.