The idea is that you swap the optical drive for the 500GB drive that you'll be taking out to replace with the SSD. Many upgraders run that configuration as a dual drive setup, manually placing OSX system files and frequently accessed data on the SSD, and placing infrequently accessed data on the HDD.
However, since the announcement of Fusion, some have discovered that OSX 10.8 will do the data management automatically if you conjoin the two drives in Terminal creating a single "Fusion" volume (just like Apple presumably will be doing in the new iMac line). In the above referenced link, a user does so on a legacy cMBP-15, in essence giving himself a Fusion drive in his laptop before the Apple version even hits the market in the iMac.
To be fair, the "Fusion" hack is new enough that you'd be a bit of a guinea pig if you did it, but I'm strongly considering it for our cMBPs.