The two Olympus digital cameras have different autofocus systems. The Olympus E-620 employs a dual AF configuration that includes a 7-point phase-difference detection system, adapted from the E-3. It focuses quickly and accurately and includes five cross target points. The system is run on a dedicated phase detection autofocus sensor, although the system switches to contrast detection in Live View or when certain lenses are attached (with 25mm f2.8, 14-42mm f3.5-5.6, 40-150mm f4.0-5.6, 9-18mm f4.0-5.6, 14-54mm f2.8-3.5II). This includes the 14-42mm kit lens, so out of the box the two autofocus systems are contrast detection. The Olympus E-P1 offers just the slower, contrast detection autofocus system with 11 areas. It's convenient since it focuses off the imaging sensor, but it doesnt perform as reliably as the Phase-Difference system on Olympus's  DSLRs. This is perhaps the E-620's greatest advantage over the E-P1/E-P2 cameras and will come in handy when trying to focus in low light or minimal contrast. 

Here's is a diagram from Olympus that shows the configuration of the dual systems in its digital SLR cameras. 

(Credit: Olympus UK)

Both cameras have Single AF (S-AF), Continuous AF (C-AF), Manual Focus (MF), and S-AF + MF. The E-620 adds a C-AF + MF mode, but it isn’t available with the contrast detection system and therefore doesn’t work with the kit lens.

Both cameras have a face detection system that works very well. Each camera can recognize up to eight faces at a time. Both cameras recognized faces quickly, and they often identified “faces” that weren’t really there.

The E-620 is much faster at focusing and getting the shot – unless its live view is activated. When the live views are used, the E-P1 is faster to focus and take a picture.