This question has come up from time to time since 2008 when Micro-FourThirds was first released. I've been working with FourThirds and Micro-FourThirds cameras since 2007, and there were similar questions then about Olympus-Panasonic compatibility with the FourThirds bodies.
On the FourThirds lens front, the differences between the four lenses Panasonic produced and the Olympus lens line are small, but curiously significant. FourThirds lenses did not include the lens correction protocol specification used in Micro-FourThirds, all lens corrections for Olympus were embedded in the Olympus image processing software (Studio and Viewer) and were specific to the Olympus ZD lenses. Panasonic at first built aperture rings onto the lens bodies (14-50/2.8-3.5, 25/1.4, and 14-150/3.5-5.6 lenses) which are soft ... the Olympus bodies didn't recognize them at all, but the body controls for aperture worked the same way with these lenses. Panasonic bodies had firmware control to allow Olympus lenses to be used with on-body controls for the aperture. Another interesting twist is that three of the four Panasonic lenses (all but the original 14-50/2.8-3.5) include the full lens correction parameter suite used in Micro-FourThirds in their firmware, even though no FourThirds SLR bodies read it ... the Micro-FourThirds bodies read it and use it.
The Micro-FourThirds mount protocol specification articulates parameters for geometric corrections and lateral chromatic aberration corrections. These are part of the coordinated standard between Panasonic and Olympus. The lenses provide these parameters to the bodies. It's up to the bodies to accept and use them.
- Olympus lenses have (until now) supplied only geometric corrections according to the protocol specification. This might be changing with the latest generation of lenses.
- Panasonic lenses have supplied both geometric and CA aberration corrections according to the specification.
- Olympus bodies (until now) have accepted and implemented using only the geometric corrections. This might be changing now with the E-M1 and later bodies.
- Panasonic bodies have accepted and implemented using both sets of corrections.
Olympus image processing software (Studio and Viewer) has always included both geometric and CA correction information for the entire Olympus lens line, the Micro-FourThirds lenses were added to this. Olympus has now additionally embedded that lens correction library into the E-M1. This library is what's responsible for diffraction correction and other per-lens special optimizations. They do not include Panasonic, Sigma, Voigtländer or any other lens families in it, as might be expected.
My personal use has been the Panasonic G1 and GF1 bodies with Olympus FT, Panasonic and Panasonic/Leica FT and mFT lenses from 2008 to 2010, and in the past year the Olympus E-PL1 and now E-M1 bodies with Olympus FT and Panasonic, Panasonic/Leica mFT lenses. I've had no problems at all using the lenses and bodies interchangeably. Some lenses are more dependent upon the lens correction parameters than others. For instance, the Panasonic 20mm used on an Olympus body corrects geometrically but not for CA, so I need to add that in LR. The Macro-Elmarit 45/2.8 ASPH OIS is interesting as it supplies both geometric and CA corrections but both are set to null ... neither make bodie does any corrections when using it.
The larger set of corrections/filtering improvements for Olympus lenses will be performed by the E-M1, but I wouldn't make the lack of them into an issue for using Panasonic and Panasonic/Leica lenses. The Panasonic 14mm f/2.5 and Panasonic/Leica Summilux-DG 25mm f/1.4 ASPH both perform beautifully on the E-M1, despite that I need to apply lens CA corrections manually (at least with the 14mm ... haven't seen any CA with the Summilux). I'll probably acquire the Macro-Elmarit 45mm, the Summilux 15mm, and the Nocticron 43mm as well over time, as I have found the Panasonic/Leica lenses just "do the right thing" for me visually.
G