I have this kind of setup in my Mac mini, 120 Gb SSD, 320 Gb HDD. Instead of doing mounts or messing with the location of the home folder I decided to just link the "heavy folders" back to their location on the HDD :
$ ls -l
total 1304
drwxr-xr-x 17 tyr admin 578 Apr 17 01:51 Applications
drwx------+ 6 tyr staff 204 Apr 21 20:20 Desktop
lrwxr-xr-x 1 root staff 35 Dec 31 08:15 Documents -> /Volumes/MiniHD/Users/tyr/Documents
lrwxr-xr-x 1 root staff 35 Dec 31 08:19 Downloads -> /Volumes/MiniHD/Users/tyr/Downloads
lrwxr-xr-x 1 root staff 33 Mar 18 16:11 Dropbox -> /Volumes/MiniHD/Users/tyr/Dropbox
drwx------@ 56 tyr staff 1904 Apr 18 03:49 Library
lrwxr-xr-x 1 root staff 32 Dec 31 08:19 Movies -> /Volumes/MiniHD/Users/tyr/Movies
drwx------+ 6 tyr staff 204 Feb 19 04:35 Music
drwx------+ 25 tyr staff 850 Apr 21 16:24 Pictures
drwxr-xr-x+ 88 tyr staff 2992 Apr 8 11:17 Public
drwxr-xr-x+ 3 tyr staff 102 Dec 31 08:02 Sites
lrwxr-xr-x 1 root staff 41 Dec 31 08:30 VirtualMachines -> /Volumes/MiniHD/Users/tyr/VirtualMachines
drwxr-xr-x@ 2 tyr staff 68 Jan 11 19:51 dwhelper
drwxr-xr-x 2 tyr staff 68 Dec 31 08:54 lf5
I've had this setup for a couple of months now and have not had any problems with it.
2 folders are not linked : "Music" and "Pictures" because I've changed the location of the music and pictures in iTunes and iPhoto respectively. For iTunes this allows me to keep the metadata (the iTunes "db") on the SSD where it is fastest.