Only one vendor, OWC (MacSales), ships a compatible SSD, but of course you can't use it in non-Apple laptops which have a standard SATA connector.
Picture in article here: http://www.mactrast.com/2012/11/apple-ssds-and-the-fusion-dr...
It is the one big thing that makes me hesitate about buying an rMBP. SSD's are prone to failure according to many articles I've read and they also have a limited life span even if it doesn't have any defects. Once the thing gives up there is no way to easily swap it out and just insert a new one.
Also, sometimes I need to swap out hard drives (or SSD drives in this case). With the rMBP that is not possible. The rMBP is a nice computer but I really hope they see the error in their ways and add a replaceable standard SATA SSD.