its not just flash sites that causes it. It can't be a heat thing with the batteries because the ones we have tried have been stone cold and within seconds of plugging one it, the fans ramps up. It will also do the same thing if starting the laptop from cold, i.e. left off overnight.
actually its not flash at all. If I take the battery out and visit the same pages, the fan doesn't ramp up. It isn't the battery though as I've had it checked by Apple, plus we've tried various batteries and power supplies. Bottom line is, if the battery is on and you visit a site with flash or video, fans will ramp up to 6200 rpm. Take the battery out with just the power supply whilst still on the same page and fan will go right down.
bit of an update to this. looking at activity monitor the fans are coming on when there high cpu usage, but the usage varies by browser. A good example is the homepage of Plex, www.plexapp.com. Nothing fancy about this page but it will cause the cpu to max and thus the fans: FireFox - 100% cpu and max fans Safari - 60% cpu - quietest Chrome - 15% cpu although there in another process called Google Helper that runs at 95% - max fans In fact, the same page wil cause high cpu load on my iMac, but the fans won't come on.