MacBook – Replacing optical drive in macbook with SSD: does it matter where the SSD drive is installed

I have this kind of setup in my Mac mini, 120 Gb SSD, 320 Gb HDD. Instead of doing mounts or messing with the location of the home folder I decided to just link the "heavy folders" back to their location on the HDD :

$ ls -l
total 1304
drwxr-xr-x  17 tyr   admin     578 Apr 17 01:51 Applications
drwx------+  6 tyr   staff     204 Apr 21 20:20 Desktop
lrwxr-xr-x   1 root  staff      35 Dec 31 08:15 Documents -> /Volumes/MiniHD/Users/tyr/Documents
lrwxr-xr-x   1 root  staff      35 Dec 31 08:19 Downloads -> /Volumes/MiniHD/Users/tyr/Downloads
lrwxr-xr-x   1 root  staff      33 Mar 18 16:11 Dropbox -> /Volumes/MiniHD/Users/tyr/Dropbox
drwx------@ 56 tyr   staff    1904 Apr 18 03:49 Library
lrwxr-xr-x   1 root  staff      32 Dec 31 08:19 Movies -> /Volumes/MiniHD/Users/tyr/Movies
drwx------+  6 tyr   staff     204 Feb 19 04:35 Music
drwx------+ 25 tyr   staff     850 Apr 21 16:24 Pictures
drwxr-xr-x+ 88 tyr   staff    2992 Apr  8 11:17 Public
drwxr-xr-x+  3 tyr   staff     102 Dec 31 08:02 Sites
lrwxr-xr-x   1 root  staff      41 Dec 31 08:30 VirtualMachines -> /Volumes/MiniHD/Users/tyr/VirtualMachines
drwxr-xr-x@  2 tyr   staff      68 Jan 11 19:51 dwhelper
drwxr-xr-x   2 tyr   staff      68 Dec 31 08:54 lf5

I've had this setup for a couple of months now and have not had any problems with it. 2 folders are not linked : "Music" and "Pictures" because I've changed the location of the music and pictures in iTunes and iPhoto respectively. For iTunes this allows me to keep the metadata (the iTunes "db") on the SSD where it is fastest.

1. The OptiDrive and similar products work very well. I have the similar Date Doubler (OWC) installed in my 2009 Mac Mini and am very pleased. The combination of the fast but small SSD and still having the larger but slower HDD is the best you can do until prices come down. Optical media is passé, as you can see by Apple now systematically dropping it from its product line, and you can easily get by with only an external drive for occasional use.
2. Yes there's a limit on the number of write cycles on an SSD but it is mitigated by the firmware on newer drives which have complex "wear levelling" algorithms (basically evenly distributing writes across the disk to reduce wear) and most (though not all, check the drive specs) manufacturers ship drives with about 10% "hidden" reserve space that gets put into operation as bad blocks fail (so called over-provisioning.) Further more there are some measures you can take yourself to limit writes to disk, such as the mounting your drive with the noatime option (see"Optimizing MacOS X Lion for SSD") which will tell the OS not to update (write) the accessed time every time you read a file.
3. Yes OWS sells an enclosure that will take a mac Superdrive. I'm sure others can be found also.
4. You will get the most performance gain putting everything on SSD, obviously. However, this is not always practical. You may have large files (e.g. VMWare images) or large collection of files (e.g. iTunes folder) that take up an inordinate amount of space on the SSD. Personally I put everything on the SSD and then create links for those directories (Movies, Pictures, Music) to the internal HDD. Applications I recommend keeping on the SSD no matter how large because of the performance boost.

One more recommendation: when choosing an SSD do a thorough search on the internet regarding OSX compatibility. Some SSD drives do not support SATA-III speeds on OSX and yet others require a firmware update to do so. I've seen even hardcore tech nerds get bitten by that one.