I've been using Google Trends for a while now for all my client's work. I find it's great to look at historic data and pick up future trends.
Great example is a recent campaign I ran for a travel client. Through historic data on Google Trends, we noted a peak in travel search inquiries during the Christmas & New Years break. As such, we developed a strategy around capitalising future sales.
Further thinking has led me to perhaps look at Search as a future indicator for human behaviors. Could we use Search to pick up market activity weeks before it occurs? I think this could work for long purchasing decisions. Such as real estate markets, travel purchase, car purchase, etc.
Matching search with trends is not a new thing.
Google knows of a correlation and have a Google Flu Trends initiative.
If we could match Search trends and business data, it becomes a powerful tool in a rapid response strategy. A business that can be mobile and adjust according to foresight opportunities will sure benefit in today's rocky markets.