Adobe states that their focus with Lightroom is the image itself. As such, Lightroom's UI features a "Lights Out" mode, which allows the tools and pallets to fade into the background at a single click, emphasizing the image itself. Similar to Aperture, Lightroom also sports a zoom feature, allowing you to focus on a specific area of detail.
You can download the 111MB beta from the Adobe-Macromedia Labs website. Recommended system requirements are Mac OS X 10.4.3, 1GHz PowerPC G4 processor, 768MB RAM and a 1024x768 resolution display. Pricing and future release dates have not been made available.