Do you need to keep them in Lightroom? I import photos from my camera as RAW images into Aperture 2. I then weed out the rubbish, tweak any pictures that need it, then export them out as high quality jpegs and import them into iPhoto for permanent storage. I delete what's left in Aperture when I'm done. I'd follow the same process if I used Lightroom. iPhoto is my preference for general browsing of photos and works well as long as I remember to tag, but Lightroom and Aperture are much better for editing them. Of course, if you need the masters, and therefore like the non-invasive editing, then Aperture and Lightroom are the way to go.

Henry, the problem with your suggested solution is that Lightroom, like Aperture, doesn't make any changes to the actual photo, instead changes are applied in realtime when an image is opened from stored meta data, and are only made permanent when an image is exported from Lightroom/Aperture. Therefore, his images would only show their adjustments whilst viewed in Lightroom, which is fine if that's his primary library. That's really the reason I export my images after tweaking them… then I can view them anywhere, with adjustments made. I guess that's the real barrier to being able to view images from a common Library if you use Lightroom and Aperture. Double Twist for example will pull in photos from locations all over the Mac, but anything from Lightroom and Aperture won't have their adjustments applied.