Actually they do.
I just looked at the Mid-2010 iMac Service Manual and when they just install the 256GB SSD instead of a hard drive, they use Apple Part #922-9628 to "shorten" the sensor, they just call it a jumper and it's this one:
You can get it for around $19 and it's Apple approved and it will not void your warranty.
If you're replacing the hard drive, this is the only part you need.
If you're adding an SSD in addition to the primary hard drive, you will need these three items:
922-9485 - Pressure Wall, Optical/SSD/MXM
This is the caddy that Apple uses to install the SSD. It sits right between the HDD and the optical drive.
922-9538 - Cable, SSD HDD Data
This cable is at a 90º angle and is small and the exact length to go form the S-ATA connector to the Pressure Wall caddy where the SSD will go. No extra cable lying around.
922-9531 - Cable, AC/DC Power/Backlight/SATA, SSD
This will provide power to the SSD, the HDD, the SuperDrive and the Display. A single cable without the need to install a S-ATA splitter.
All three parts are $19 each, so you can do the official, non warranty-voiding upgrade for only $57 bucks and you can leave it if you ever need to take it for repair. You need to take out about 30% of the computer to install it and if you know what you're doing, you can be done in less than an hour. It's just basically removing part of the logic board, installing the pressure wall, hooking cables and putting everything back together, nothing out of this world.