I recently purchased a 120GB SSD that will replace my optical drive in my MBP. It also has a 500GB 7200RPM hard drive.
Is it possible to have a single logical volume that uses both physical disks in Lion. If so, would I be able to still specify that the OS and Applications be installed on the SSD while all other large files reside on the HDD, and how would I do it?
You certainly can make two or more physical drives into one volume that the OS sees. It's referred to as a concatenated disk set - and Disk Utility will erase both drives in preparation for a RAID - you can choose an actual RAID stripe or simple concatenation.
Since Macs won't boot from a RAID volume without some additional hardware or extra software ( to handle the added complexity that RAID entails ), you won't be able to use this for your boot volume.
Even if you could, I wouldn't advise doing this since the OS decides where to store files inside any volume. If your goal is to control what files go to the SSD and which to the HDD - this doesn't get you any closer to that goal.
The good news is there is an excellent "Plan B" for you
The best writing I have seen recently on how to both think about using a SSD with a HDD that also covers how exactly to implement a split was written recently by Matt Legend Gemmell - He really knows his stuff in general and his article titled Using OS X with an SSD plus HDD setup is great reading.
Matt advocates
I hope that gives you a better solution to what I see as the major thrust to your question. You might also look at using Aliases (OS X Finder's enhanced version of a sym link that points at unique characteristics of the file as well as the path in case either gets moved and the system can figure out (re-link) to the correct/new location of the file.)
As a side note, I had the same set up, and I was a little annoyed that I couldn't put my MBP into hibernation any more. The apparent reason was that the system disk (the SSD) was attached to the SATA port which was originally used for the optical drive. After swapping the disks (SSD on the primary SATA port, HDD on the secondary), hibernation worked again. So, if you want to be able to hibernate, put the SSD into the primary port and replace the optical drive with the HDD.
You would mount a certain subdirectory (eg. /Users) on the other disk. Check out http://lnx2mac.blogspot.com/2010/09/moving-os-x-users-to-separate-partition.html for detailed instructions.
Since you are manipulating the FS, make a backup first!
I don't see why a concatenated configuration via Disk Utility wouldn't boot (I also don't see why an Apple RAID card controlled configuration wouldn't boot). I have successfully configured a concatenated boot volume using a Mac Pro via the OS X Disk Utility (however, none of those disks were solid state)
Otherwise, Plan B as mentioned before sounds best. In regards to symbolic links (which I only just read about), have Mac OS refer to your user's folder on the larger volume (the hdd) via System Preferences > Accounts > Advanced. Afterward, create a symlink for what would be the "user/library" folder, taking advantage of performance on an SSD while making good use of capacity on an HDD.
