With some Macbooks it is possible to remove the DVD drive and install a second HDD. You need to buy a slimline SATA adapter to attach the HDD to the DVD power cable.http://www.amazon.com/Micro-SATA-Cables-Slimline-Adapter/dp/B003YVP8VS/ref=aag_m_pw_dp?ie=UTF8&m=A1UCLUF7KW7AYG
I've installed a 128GB Vertex 4 SSD and a 750GB HDD Drive in my Macbook using the above method. I've partitioned the HDD into a 128GB partition and another partition with the remaining space (actually there is a small 100mb boot partition on each drive too). I've then setup a Hybrid SSD/HDD RAID 1 array as described here http://www.tansi.org/hybrid/. This method gives you the read speeds of an SSD with the reliability of RAID 1.
On the RAID device i have my system/home filesystems, and then on the big non-RAID partition on the HDD i've got my big media files/data files etc. This setup is really nice.
Once thing I've notice though is sometimes when a write happens to the RAID device i get some IOwaits if i am reading from the non-RAID part of the HDD. This tends to be noticeable when playing a movie. So to get around this you can set the read-ahead for the device using hdparm. Eg hdparm -a 1024 /dev/sdb. This seems to fix the problem as enough data is present for the movie player to keep reading data while the IOwaits happen.