The data is out there:
Where did GeniuSxBoy get the number of people currently connected to Mt. Gox live?
https://bitcointalk.org/index.php?topic=31614.0I think the max number of people connected to the various live charting sites during the recent craziness would be a pretty good indicator. I am not sure whether this data is available or not though.
Lower left corner of Goxlive shows the currently connected: 430 right now. If you sum all the (known) live charting sites, it wouldn't be every trader. Those so inclined to do so, will write their own, along with the bots they use. I am about to write my own personal app for it since the sites are definitely suckin'. Though it would still run on the same API the the others run on, so it may not be more reliable anyway.