Just some follow up, perhaps prematurely. I appear to have successfully added the M500 (250GB) SSD to my 2010 iMac. this is a long post, but I post it in case folks are afraid to crack open their iMac. I would do it again, though I pray that I will not have to.
the parts I was most nervous about, the glass removal and display removal, were easy. I wore gloves through this stage to reduce risk of fingerprints, and a light dusting with canned air before re-assembly was great. I used a combo of iFixit's directions and OWC's directions. OWC just has a video, but it's very helpful and provides good visual advice how to store the glass and display inside of the iMac's box. they differ on where to place the drive, and I'm unsure I did the right thing (I placed it under the DVD drive as shown in iFixit). OWC recommended putting it closer to the existing HDD. OWC's way makes sense from an airflow point of view--I think I may be partially blocking the graphics heatsink with the drive and its cables.
I used an OWC installation tool and parts kit (bought from amazon), and it was excellent. the forceps in particular were a godsend when dealing with the display screws (magnets and stuff made that hard), and forceps were also great when dealing with small connector wires.
I think the very hardest part of the install was after I had everything disassembled, and it was time to route cables. The OWC cables were perhaps too bulky, and I wasn't sure I did a good job--air flow may be impacted. I include the part where one puts the main board back in as cable routing, because things were loosey goosey until I was able to get that fastened back in (cable bulk made it difficult to get the graphics heatsink to drop back in. there was a too-small cutout in the wall between where the SSD was and where the HDD is (the iMac has sealed off areas for air flow, so that the graphics's gets all the airflow from the right sided fan I think). the DVD drive at first didn't fit nicely with the SSD cables, but it eventually did (I think I was just doing it wrong, rather than smushing the new cables down and out of the way).
surprisingly, it was easy to get the connectors back into place except for the wifi card cable--it is flat, fabric covered, and has a 90 bend and is exactly the right length it needed to be, pre-SSD. the extra power cabling from the OWC kit was bulky and the SATA cable for the new drive was routed through the same area. It made the wifi card cable a tighter stretch than I would have liked. but ultimately it came together and I'm posting this from the iMac, on wifi. (I usually have this mac on ethernet anyway, in case the wifi cable comes out later--it's a loose connection held on by tape, tape that I tore/weakened).
about 4 hours to do it (yikes). half hour to get all the tools lined up and to get my area ready, an hour and a half to take everything apart, another hour and a half with the cable arranging and first re-assembly steps (cable routing as main board being put back, getting that board back OK, getting that airport card cable plugged back in, and another half hour for the remainder of the reassembly.
so in a nutshell, that was my SSD installation. I purchased winclone a few days ago, and made an image of my windows 7 bootcamp partition, saved to the desktop (in a folder that's not time machined--I don't want to fill up my time machine with this file). this image was backed up to crashplan so I'm not total reckless. As I understand, my next step is to make a bootable USB flash drive with mavericks, so I can go do the DIY fusion drive instructions. I'll do this another day. for me, the big barrier was cracking open the iMac.
on a side note--has anyone done this install and then had thermal problems, like can the power/SATA cables melt from the heatsink (this mac can get HOT when running a game or something that needs graphics.
thanks! I'll post back impressions after I do the fusion drive stuff.