Actually there's a website called Herd Tracker run by a guy called Carel which is fantastic for tracking the wildebeest. I've been following it during my 3 months in Africa.
its not useful for long term planning because the wildebeest move erratically, but if you're booking just before you depart then you'll know where the animals are.
You're right there were wildebeest crossing the sand river backwards and forwards since June, however right now most of them are clumped around the Mara river in northern serengeti due to better than normal rain in that region.
I'm planning to do a Mara safari before my plane leaves Nairobi on 24th so I'm hoping they get a move on
