Well, you have a few choices.
The most "straightforward" is to use the Lightroom "Edit in Photoshop" function. You can do this by opening as a Smart Object, which has some different "characteristics" and can, I believe, open the Raw file directly, although my version of Lightroom and CS/ACR don't "match", so I'm not sure of how it works with the matching versions of CS5/LR3.
The "normal" approach, though, is that your image is converted to a tiff or psd, according to your Preferences. This file/copy is automatically saved to your folder alongside the Raw file and is imported into your Library so that it will either show up with the Raw file or will be "stacked" with the Raw file, again as determined by your Preferences.
When the image opens in Photoshop it will show the LR processing. Then, after doing your Photoshop work, you have choices: you can Save your image and it will overwrite the tiff/psd in your LR library, and so the updated image will "show up" in LR. Or, you can use Save As to create a new version that will not automatically show up in LR, although you can use LR to import the new version if you wish. Or, you could Save for the Web (or some other device) and create a jpeg, for example, for some output purpose. This copy will not be automatically imported into Lightroom. In fact, many Raw shooters don't want output jpeg copies cluttering up our libraries.
And then, there are alternative ways of using Photoshop. One straightforward way is to Export, and then to use the Open in Photoshop option in the Export dialog (in the Post Processing panel and the After Export drop-down list). This would be the option to use if you wanted to work on an image in Photoshop without it being automatically included in your Lightroom library. This could be handy if all you wanted to do is bring it into Photoshop and output say an image for the Web or for printing and be done with it.
You can also run a handy little thing called a Photoshop "droplet" from the Export dialog. This is where a Photoshop Action has been "wrapped" in a way that makes it "callable" by an outside app or the system and typically an "in and out" action where the image is opened in Photoshop, processed, and then configured to save and close without user intervention, or at least with minimal user intervention.
And then, there is the more hands-on approach. Since you are saving your metadata in xmp files, and since your version of Camera Raw is compatible with Lightroom 3, you have the option of going to Bridge and having the LR metadata "visible" to Bridge and the Camera Raw plug-in. This can give you the ability to either open a Raw file in Camera Raw and then into Photoshop with all the LR edits applied, or to take advantage of some of the tools Bridge offers for Photoshop, again with the LR edits applied. For example, you can use Bridge to open a number of files and invoke something like HDR or Panoramic stitching, or to open them as Layers in a stack for blending in one way or another.
Note that you can also open multiple files in Photoshop from Lightroom. In fact, Lightroom shows options for merging, etc, although I'm not sure how I work because for some reason my LR3 doesn't have them enabled for CS3, and that may be the problem. Give it a try -- select several images and right-click one and go to the Edit In list and see what your options are!
Well, those are some things off the top of my head, hope this helps!