How many people use Google Trends ? It doesn't seem like it would be tough to do a sort of Google-bombing, or trendsetting with Google trends. You could apply the same <img> concepts cookie stuffers use, you could referer spam with a SERP address, botnets could tack on to the query count. What if you used this to dupe your competitors into wasting resources on new trends that aren't really even trends at all ?
Well, you could give it a try - spend a lot of time and energy trying to second guess google's ability to spot when their system is being manipulated, in the hope that your competitors use Google trends, use it in the way you think and will then act on what you've tried to manipulate. or you could spend your time on improving your own promotion and site content. I wonder which would be more useful? </sarcasm tag>
Anyone can spot their system being manipulated, no matter how "fool proof" something is, manipulation always leaves behind traces of itself. The real question is how long will something go on before the manipulation is noticed ? Feedburner stats have been getting gamed for months at the very least, I bet trends would go just as long if someone were to game it. Google, especially now, has to focus anti-gaming efforts on search results and the Adwords program. Googles bottom line wouldn't change at all if Trends were to vanish tomarrow.