different than normal platter-based, the individual cells on the SSD drive will only hold data as long as they aren't overwritten which means that you would only need to do a one pass erasure on the free space and there is no chance to get that data back.
as it is well know, SSDs do have this reliability concern and limited writes but it would seem to be personally that doing a single pass overwrite of free space once a week or whatever woudln't have an noticeable impact.
On the other hand, its the NAND controller who is dictating where the data will be written so even thought the OS things is overwriting Free space, the NAND controller is writting somewhere completely different.
Best thing in my opinion to do is to use encryption.
Over the phone, Intel recommended using the HDDerase utility to clear a drive. I haven't tested this yet, but supposedly it uses the ATA "secure erase" function to do the job. This might ensure all bits are zeroized, especially if the SSD might have 'extra' memory to recover from bad ones. I'm not convinced the X25 M series has these extra bits, but the E series does.
Also, you can overwrite with any other utility, and as mentioned you'd probably just need to do it once. Chances are, your data won't be recoverable by any non-government funded entity after one pass. I was hoping Intel could provide a utility which would erase and 'trim' the drive back to factory condition, but I think the average user wouldn't notice any difference in performance during *regular* daily usage.
Does this help?