My MBP from late 2008 has a chinese branded optibay (11$ plus 3$ shipping!) and it holds my 7200 rpm 500GB WD scorpio black, in my hardrive bay is a 120 GB vertex 3 ( I know its sata3 but it runs fine on sata 2 too) and I have not had any problems at all, according heat and so on.
The whole process took me like half an hour..
I have changed them around SSD in optibay and HDD is normal bay, but so far I have not noticed a difference..
My data (~/Music ~/Pictures ~/Downloads/ is on the drive and the OS on the SSD, all is superfast! Booting takes 11 seconds..
All Applications are "instant on" and my brothers i7 MBP with HDD from 2011 feels a lot slower.
I have ordered a sata to usb converter thingy and a micro sata cable for my superdrive, so I can still burn the occasional DVD/CD when needed.
Best upgrade ever!!