Olympus published a roadmap for µ4/3 some months ago (I think it was at the end of 2009). If Olympus follows their own roadmap, they will introduce a macro lens in September (maybe at Photokina). Most probably it will be a 50mm lens as suggested by the roadmap.
This question seems to pop up fairly often. As far as I am concerned, the 45mm 2.8 Lumix is a wonderful lens. It is sharp, handles well, and doubles as an excellent portrait lens. I know it's a bit on the expensive side, but worth it in my opinion.
In terms of legacy lenses I have used both the 55mm and 105 mm micro Nikkor lenses, but my lens of choice is the Sigma 105mm, f/2.8 DG Macro in a 4/3 mount with the DMW-MA1 adapter. I use this with a Panasonic G1 which retains the auto stop down feature, but not the auto focus. I don't know the details of the compatibility of this set up on an E-P1. I find that this focal length works well with the more flighty insects by giving you greater working distance. If your interest is more towards flowers and static subjects then the 45mm/55mm focal length lenses are probably more suited.
I would agree fully with Antonio, but with a (big) catch. I found that the autofocus on the E-P2 was unreliable: slow and iffy. It tended to hunt for a while, even when the focus range was on the Limited setting, and sometimes stubbornly refused to settle on a focus at all. I never really warmed up to using it as a manual focus lens on the E-P2, since the art of focus-by-wire seemed to elude me.
For my part, I had kind of given up on using it either as a macro lens (I preferred my old OM 50/3.5 macro, which I first purchased in the early 80's) or as a portrait lens (I preferred an OM 50/1.4, which I picked up for a reasonable price on eBay). I still liked it as a medium-reach tele lens, and used it a lot as a walking-about lens. (Not a surprise, since my favorite focal lengths in the old film days were 85mm and 100mm, and this one--at 90mm equivalent--is smack in between those.) It's undeniably clean and sharp, which is why I hung on to it.
All this changed, I need to add, when I switched to a G2. Suddenly the lens came into its own, with the autofocus snapping quickly and surely onto the target. A quick touch on the screen will also let me switch to a new target, and it seems to go there without any hunting at all. To my surprise, even manual focus is more comfortable for this lens on the G2, with the camera popping up a focus range indicator that reinforces the focus-by-wire movements.
I realize that you're asking about the E-P1, though. As I recall, Antonio also has an E-P1, so he'll probably be able to give you a better feedback on how this lens will work with your camera.
I have had no difficulty selling images taken with the Oly 35mm f3.5 lens. You need to be aware that you have to get VERY close to get a magnification any where near 1:1 which may make it not suitable for your particularr needs.
I'm using the Lumix 45mm 2.8 macro on the GF1. On the EP2, I have a 50mm f2 Oly 4/3 macro (which I got at a great price) attached with an adapter. Both are great lenses! My pics are all tagged on Flickr by lenses, so if you want to see results, please take a look. BTW, the 4/3 Oly macro autofocuses on the EP2 with the chipped Lumix adapter. JP's Flickr stream
However the 35mm lens offers a maximum image magnification of 1.0X as compared with 0.52X with the 50mm which you would need to take into account. The 50mm is form olympus Pro Range and the 35mm from their standard range. So the 50 will have better dust proofing which may be important to you.
I've just tried my OM 50mm f/3.5 macro as well -- my hand isn't as steady as neilvan's, but I've still had some great results.
The nib in the second shot seems to be at an angle from the lens's focal plane. Oh well.
Taken with non-ideal lighting on a restaurant table. First two are unretouched, third is retouched. Color was pretty accurate in the original and the retouching makes it less so, but it makes the nib contrast more with the background.
Camera: Panasonic GF-1
Lens: Olympus OM 50mm f/3.5 Macro, hand-held (not my usual practice for macro)
The linked images have a pixel peeper for you folks that like that kind of thing.
Neil, welcome to Serious Compacts - that is a truly beautiful spider photograph. I am close to an arachnophobe but can still appreciate their beauty via a photograph and this one seems more benign to my eyes...perhaps due to its coloring and that beautiful blue/periwinkle background. If you haven't yet, please do check out this thread https://www.photographerslounge.org/f20/show-insect-471/ and add to it as you'd like.
Deirdre, thanks for your macro shots here, too. I'll be looking forward to more.