There is Voigtlander 40mm f/2 pancake for Nikon mount. Metering works with the small nikons too, and A and S modes. Nikon also has Nikkor 45mm F/2.8 pancake lens.
Also you can get the really small nikkor 20/4 AI-S or slightly larger nikkor 20/2.8 AI-S for compact wide angle needs.
I said "DX or EF-S" lenses for a reason: for DX bodies those 40mm amd 45mm pancake lenses give far narrower than normal FOV, so only the (non-pancake) 20mm options are relevant in comparison to the normal FOV of the Olympus 25/2.8 pancake. The Nikon 20/2.8 is 42.5mm long and weighs 270g (I cannot find the spec's for the 20/4) compared to about 25mm and 95g for the Olympus 25/2.8. Other Nikon options include the 24/2.8 and 28/2.8, closer to normal FOV in DX. The 28/2.8 is lighter than the 20/2.8 at 205g, but the 20/2.8 is the shortest "near normal" option.
Also, the Nikon F mount has a distance of about 46mm from flange to focal plane, compared to about 38mm for FourThirds mount. So the distance from focal plane to front of lens is about 63mm for a FourThirds body with 25/2.8 compared to about 88mm for a Nikon mount body with the 20/2.8. So the Nikon options do not come close to the low camera depth and weight of the E420+25/2.8.
P. S. I found the specs for the Nikon 20/4. Apart from being a long discontinued manual focus lens, it is longer still at 47mm, but lighter than the 20/2.8 at 205g (still over twice the weight of the Olympus 25/2.