I am doing research on Twitter trends and defining them in general, in order to develop better algorithms. So this is very timely and instead of starting a new thread, perhaps we can expand it a bit.
There's quite a few services doing twitter trends already, and most are doing it with a simple pragmatic approach as defined by David above. I wonder if the operators of those services could say a few words here about their approaches. If we keep an overview of the field and develop ways to compare and evaluate trend-tracking algorithms, it will become science and will work better. Cheers, Alexy