Just to add on to the original question from the TS, which many answers have missed, Lightroom is a photo library while Photoshop is a pure photo editing software. Many photographers neglect the importance of a good library software to help manage your huge numbers of images (and now videos with HD recording in DSLR). I for one have enjoyed iPhoto for many years now and understand the importance of letting a software do the arrangement of images and folders, so that i dont have to worry about creating folders, naming, searching, etc. When comparing photo library functions, I feel Lightroom is still inferior to Aperture (or even iPHoto). With Aperture or iPHoto, i can truly not worry about where my files are kept. The software manages it for me, and with Aperture, even the backup of master files. Lightroom still need the user to intervene and set folder names and rules during import. and a poorly thought through folder planning can lead to disaster in the future, when library gets too big.
Having said that, Ligthroom is also a photo editing software, with all the ACR capabilities built in. The only (and probably major) difference is that it cannot do layers, hence colour ranges, masking, overlays, etc would have to be done in Photoshop. Another thing which makes Lightroom more powerful than Aperture is the new lens correction feature. It have many preset lens profiles, with ability to manually adjust. This is very useful for UWA lens's defects. Another thing working for the newer library software: all edits are done non-destructive and no additional image files are created. This means you save harddisk space (if you are like me, keep RAW+JPG), and you can change the image editing anytime.
Someone mentioned Photoshop Elements. It is the stripped down version of PhotoShop CS. I think it is already quite good, considering the price you pay, and if you dont need the full suite like illustrator, page maker, etc. I tried the evaluation version, and found some features deliberately removed in the PSE version compared to the full Photoshop CS.
At the end of the day, what are you looking for? A library or an editor?
Unfortunately for me, either way, I won't get Lightroom. I'd get Aperture for a much better library and PSE or Photoshop CS for a full on editor, + PT LENS plug in for lens correction. That is just my preference.