Making good use of HD + SSD combo on a new iMac I am setting up a new iMac that has a 2 terabyte hard drive and a 256GB SSD drive.  The Operating System came installed on the SSD drive. 

My intention was to put the OS on the HD based on its longer potential lifespan and use the SDD for things like the Photoshop scratch disk.  Getting things set up this way at this point would seem to require me to re-install the OS and specify the HD, which would not be the end of the world because I  have not done much configuration yet.  

Are there other options I should be considering here?  My goal is maximum long term reliability, but I do also want to take advantage of the SDD's strong points.
My first advice: don't fret too much over SSD lifespans or performance degradation. While it has been an issue (and still is), for almost everyone it's not likely to be an issue over the lifespan of your computer. Far more likely that you'll want to replace your iMac before the flash runs out of writes. Additionally, the Samsung and Toshiba drives Apple currently uses are are more in the reliability end of the spectrum, rather than sacrificing lifespan for huge speeds.

Secondly, to really get the most benefits out of the SSD, you'll want to put your OS and apps on the SSD, and big files like movies, music and photo libraries on the HD (if you keep the metadata files like iTunes Library.itl on the SSD, you get speed improvements opening big libraries, without having to store the big files there). The most noticeable improvements from SSDs aren't the huge sequential r/w speeds, but the smaller random access speeds, which you really notice for OS and app start times, and any memory paging to disk.