Let me add a couple more details to my earlier reply. This is not to contradict Jeremy and John, because certainly Lightroom is a very fine program that Adobe has given a lot of thought to, and is one that many photographers find works very well for their needs.
LR's development features, the raw conversion and editing tools, are based on the same software engine found in Adobe Camera Raw--my understanding is that "under the hood" the two are identical so that you can perform the same edits in either program. The user interface however is somewhat different, with ACR being a standalone module opening into PS while LR's development features are integrated more closely with the file management tools.
Bridge serves the same basic functions as the file management tools of LR (downloading files, renaming, indexing, sorting, searching, retrieving, and managing batch operations) but under the hood, the two programs are completely different. LR has a true database management program that allows very powerful and fast management of large groups of files, including offline storage. Because the database and development tools are combined in one program, LR makes it especially convenient to work with groups of files and apply edits to groups of files, such as correcting the white balance for a whole day's shooting. If all or most of your editing needs can be met with ACR, then it may be that the convenience of LR would be worthwhile--it would really be a question of user preference. Obviously, the question of user interface is one on which people may have strong feelings--if you spend a lot of time with a program it is important to have something that works for you. Whether that is LR, or Bridge and ACR, only you can say.
Unquestionably, PS provides a host of editing tools not found in LR and ACR. Whether you need them, only you can say, but if you want those tools there isn't really any substitute for taking the image out of either LR or ACR and working on it in PS. The versions of Bridge and ACR that are included with CS4 are greatly improved from those with CS2. ACR has a number of important new features and Bridge was upgraded from CS3 to CS4 so it seems to work much much more smoothly and has an improved interface. My own opinion is that PS CS4 has provided a lot of other improvements of particular use to photographers compared to CS2, so if you need PS, the upgrade to CS4 is very worthwhile whether or not you use LR. For me at least (I do a reasonable volume of shooting for an amateur) the tools in ACR and Bridge are more than sufficient and I couldn't justify the additional expense of LR unless I had a much higher volume where I needed the more sophisticated database of LR.
Hope this helps.
