OK, I'm watching this show on Discovery, where these scientists theorize that the Earth was completely encased in ice about 600 million years ago.
The first guy who thought this, and this was in the 40s, based it on the existance of dropstones (stones dropped by glaciers when they get all warm) in Namibia. During that time period, the theory of continental drift and tectonics were not accepted theories. So, they thought the continents had always been where they are now. In fact, part of Wegener's basis of continental drift was the apparent glaciation of Africa and South America. So, during the mapping of magnetic abnormalities (I believe this was in the 60s) they discovered that the continents are, in fact, 'drifting'. So now this became the argument against the snowball earth theory, and scientists continued to accept that the tropics could not have been glaciated.
So, this one guy from Cal Tech, he took a highly sensitive magnetometer and got the magnetic orientations of the dropstones in question during the time of their formation. His data indicated that these rocks were formed at the equator. And Shazam! Proof positive that there was glaciation at the equator! But wait... let's think about that. The only time in which a rock will acquire a magnetization is during crystallization (or diagenesis for secondary magnetizations). Once the rock has crystallized, the magnetizations are in place forever, or until the rock undergoes complete recrystallization. So, what he actually discovered is that those rocks were originally created at the equator. There's nothing in the magnetization data to indicate that the rocks were picked up and deposited by glaciers at the equator! They could have been created at the equator, then uplifted to glacier heights, and THEN transported and dropped by the glacier. Glaciers *do* form between 30 and 30, but only at really high altitudes (e.g., the Andes mountains in South America).
Now, granted, this was a tv show on Discovery, and not the actual papers involved, so some data which would be important to me as a scientist may have been omitted so as not to confuse the layman. I'm not saying that the Earth was *not* completely glaciated for a few million years, I'm just saying the evidence presented in the program is inconclusive.