Easy guide on how to do it:
1) Back everything up somewhere (e.g. with Carbon Copy Cloner, free/donation, to an external HDD)
2) Put SSD in ExpressCard slot
3) Use DiskUtility to format as HFS/Journaled
4) Install Snow Leopard on to SSD from the SL DVD
5) ~Fiddle with the permissions.
of the 5 tasks, only the last caused me any issues and that was because I'm a bit of a thicko at these things. I created a user account on the SSD that was exactly the same as my user account on the old hard drive, and then in the system prefs / users section right clicked on the name and altered the path to that of the old account. This did the trick but I'm sure there are better ways for those who understand these things...
A few apps needed reinstalling (SPSS, EndNote, and sadly Office for Mac 2011) which needed the old .dmg files, but basically the machine has gone from perpetual 5-10 second lags whenever I asked it to do any additional task, endless SBODs, and mild to moderate frustration to a snappy, responsive, low lag system which scoots along nicely. Especially noticeable opening Word, LaTex or iTunes - one bounce in the dock and we're up and running. Boot and shutdown times much faster. Definitely recommended. In my case I had access to all the software for reinstalling over the network locally so it was all very painless - but be wary if you don't have install discs, as I can't always run apps on the old HDD (I'm assuming this is permission related but haven't worked it all out yet).
System: 2008 MBP (4,1) 4Gb, 250Gb HDD, OSX10.6.7. This was I think the last alu model prior to the unibody MBPs. In response to various questions I have seen asked on other sites, 1) the SDD card sits out by 1mm from the edge of the machine, 2mm at the USB port bit and 2) Battery life actually seems slightly better than before - but my battery is old and needs replacing (only holds 71% health and has had >500 cycles).
Hope this helps. Happy to answer questions.