If I open a raw file in Photoshop it opens it in Camera raw. When I think I'm finished editing I click "open image." And now I'm in regular Photoshop. From this point, is there anyway to reopen the Camera Raw window? Or do I just exit the image and reopen?
What you need to do when in Camera Raw, if you think you may want to make further ACR changes, is instead of clicking the Open button, first hold down Shift and the button will switch to "Open Object". Now click it and you will get a smart object layer. You can double click on that at any time and reopen ACR and make adjustments and keep anything you've done within PS.
If you use the normal Open from ACR, then you won't be able to go back to ACR and keep your PS changes.
I've just tested this using CS5. If you open a raw file into ACR, make a few adjustments and then open it up into Photoshop, you can re-open that raw file (the usual File->Open option from the menu). ACR will start up again using the same raw file plus the changes that you made in ACR the first time. It won't include any changes you've made in Photoshop.
When you make changes in ACR and then choose either the "Open file" or the "Done" options at the bottom of the screen, those changes are saved in an .XMP file alongside the raw file (usually in the same folder, although this can probably be changed). The next time ACR opens that file it will apply those changes again. If you want to start again from scratch you just have to delete the relevant .XMP file. If you choose "Cancel" from the bottom of the screen, then the changes will not be saved - you'll just have the original raw file.
So the short answer is that you can reopen the ACR window by reopening the raw file. Unless you've clicked "Cancel" or actively gone and deleted the associated .XMP file, the changes you made the first time in ACR will automatically be applied.
One option is to save the adjusted file as a JPG, then you can re-open it in Camera Raw and it will have all the adjustments which were made in Photoshop. It will obviously be a 'flat' file with no layers, but I do this quite often because I prefer the gradient adjustment in Camera Raw to that in Photoshop and it's often not obvious that I need one until the Photoshop adjustments have been done.