Lightroom does not use the ACR plug-in! But there is a release candidate of Lightroom 2.6 available for download.
To accomplish a manual install on Windows you can copy and rename the file:
Which appears to be about the right size (11MB) for Camera RAW.8bi (32-bit).
I believe there is an analogous process for the Mac install image, but I don't know what it is. You could probably poke around in the folders on the disk-image and find it. You might check the size of Camera RAW 5.5 to get an idea how large it should be on your system.
What you can't install by this procedure are all the camera-specific profiles for newly supported cameras in this version. You would probably need to install the LR 2.6 RC to get those.
If you do find and successfully manually install the Camera RAW plugin from the Mac installer image, you might post your results, here.
The OP said CS3!
I found the camera raw.plugin file and copied it from the dmg file to the directory where the 5.5 plugin was located. No such luck. Bridge doesn't work with Canon 7d raw files at that point. Did the same thing with the camera files and they did not seem to be a problem when I copied the 5.5 plugin back into the directory (Library\Application Support\Adobe\plugins\cs3\file formats).
That tis the procedure for a manual install in Bridge CS3 and Photoshop elements 6 for versions 5.5 and prior.
I don't know if the install files does something to the plugin fle on the extract or if it requires Elements 8 for 5.6
I did download the LR 2.6 RC and it is working
On Windows, ACR 5.6 RC works with Elements 6 so I expect it would work with 7 as well.
I would search your hard-drive for all instances of the Camera Raw.plugin and replace them with the one extracted from the DMG, after, perhaps, renaming the old copy to something with a different extension just in case you need to put it back.
I have searched for the plugin and it only finds the one that I replaced. This is in the directory I mentioned which is exactly where the plugin had to be placed per the manual install instructions that adobe put out with 5.5. If there is another place it exists, search does not show it. Interestingly, get info on the file in the dmg (install file) with the new camera raw.plugin does not show it to be associated with elements. Doing it manually did not work. The icon is different as well (an all white shield as apposed to what looks like a leggo block). Not sure what, if anything that means.
It works exactly as you described.
I renamed the 1003-file to Camera Raw.8bi and replased it with the "old 5.5"
No restart of anything else than PSE.
Message came up saying ACR 5.6 is RC expiring end of january.
I have Windows7 and PSE 7.0-32.
If you do a Elements / Help / About Plugin... / Camera RAW what version does it say?
PSE Help says: Camera Raw 184.108.40.206
Graydl, what version of ACR does Elements / Help / About Plugin... / Cmaera RAW indicate?
I reinstalled the plugin the way I described and Elements reports 5.6 RC.
However, Bridge cached thumbnails are very fuzzy and if you purge a folder cache it will not rebuild them (at least with Canon raw files from a 7d) with 5.6 installed (which is one of the reasons I thought it was not working at all). So the problem is more Bridge than Elements.