Recent reports have shown that theta oscillations are in fact traveling waves along the septo-temporal axis of the hippocampus (at least in rats).

Lubenov and Siapas, 2009, Hippocampal theta oscillations are travelling waves

Patel et al., 2012, Traveling Theta Waves along the Entire Septotemporal Axis of the Hippocampus

Here is a little animation based on my 3d model of the rat hippocampus to show how the traveling waves might look like in CA1.