There are not many native micro 4/3 lenses available yet, especially large aperture prime lenses, which is basically non-existent at present. There is one 4/3 large aperture lens, the LEICA D SUMMILUX 25mm/F1.4 ASPH, but it is a rather big lens to fit on a micro 4/3 camera like the Pen E-P1 and it is also rather expensive. One of the attraction of the micro 4/3 system is that you can use a lens mount adapter to mount different system lenses to a micro 4/3 camera and Leica M mount lenses are one of the very popular choices.
I bought a Voigtländer NOKTON classic 35mm F1.4 today to go with the Olympus Pen E-P1. It is a straightly manual focus lens, but it is small and compact with a very good built quality. The retro look compliments the Pen E-P1 perfectly.
I have not had time to test out the lens extensively yet, but here is just one test shot I took at home tonight at f1.4 and ISO 1600. It was shot with RAW and converted to TIFF file with Olympus bundled RAW converter. The TIFF file was then resized and sharpened in Photoshop before converted to jpg format for web output. No other post-processing was done.