Well, here's my response to Gordon's question. Be warned, it's a little overly influenced by Malcolm Gladwell/muddled, but this question could take days to answer.
The short answer is yes, I believe that the most powerful brands are now made by consumers, in the hearts and minds of the masses.
The longer version is as follows:
Like blaiq thinks, I feel consumers have always had more of an active role in communications that some of the advertising community believes. No consumer is ever passive. I recall reading a Jeremy Bullmore speech transcription on Russell Davies' blog which emphasised this.
Malcolm Gladwell's 'The Tipping Point' makes an interesting case for people always being able to communicate in this way. Without wishing to go into too much detail, viral marketing has always been around - be it word of mouth of those whose information we trust, those who persuade us by their sales-pitch and those who we consider well connected (the 'Mavens', 'Salesmen' and 'Connectors' in Gladwell-speak). These people can help spread the word easily.
However, it is only now, with the advent of the blog and the ease with which many can communicate (such as people like myself who find HTML worrisome) that brands are being 'taken over' as it were with opinions of those who can reach more people with greater speed than ever before.
If then we accept all of this, consumers can now make recommendations (by means of Amazon, MySpace or another peer influenced network) and trends can occur ever quicker. This not only speeds the communication cycle, but it increases the necessity for the product/service marketing to be good ALL of the time. As Northern Planner/Andrew states, this can't be done. People will begin to mold brands in ways in which they cannot conceive.
Hence, consumers now make the most powerful and evocative brands.
This raises another interesting question when it is applied to conventional advertising - will agencies be now promoters of the brand's tone of voice, or less than that? I think there will always be a place for direct branded communications, be it viral, ATL, BTL or ambient.
I think the question coming out of all of this debate is blaiq's point of view once again - 'haven't they always been?' Probably. But now consumers have the means to actively shift and bring about brand change.
Brands, therefore, must engage the consumer in a dialogue; something which modern-day advertising is very keen to achieve. The likes of Innocent's blog provide a useful observation point - will making the brand 'open source' create a better brand? In the case of Innocent (whose philosophy appears to correspond with collaboration), it should.
Whether this will work for every brand is an interesting question; one in which we'll only find out some 20 years from now, I think.
So yes, the most powerful brands ARE now made by consumers. Whether brands/advertising agencies will seek to reclaim their mystique remains to be seen - indeed, whether they can once opened up is a fascinating debate.
Anyway, enough of my random wibblings. Read Russell Davies's debate about blogging (and the comments) for another perspective.