"I assumed if I left it alone, the installer would, too." - By specifying partitions manually, you have instructed the installer that you wanted full control over the partitioning process, and you can't really blame Ubiquity for not reading your mind. It's not safe to assume anything when undertaking a potentially risky process such as messing around with your partition table, just keep that in mind.
Anyways, you have a couple of choices. Either locate your old /home partition (my guess is either sda5 or sda7), copy the contents over to your new /home folder in your Mint 9 installation, and ensure that afterwards all permissions are correctly configured (chown and chmod are your friends). Or, you can choose to mount your old /home partition within your new installation; auto-mounting at boot would probably require a bit of tinkering with /etc/fstab though. The 3rd alternative is simple; just run the installer again and this time, make sure you get your choices right during the partitioning step.