When taking apart laptops, I find that the designers put case screws in some pretty funny places. I recommend taking off things like the cover for the RAM, and looking under it for case screws. I'd also look under the battery and inside the battery compartment, they like to hide the case screws in there.
In general, I've found that, when the case won't come apart with light pressure, there's usually a screw somewhere nearby that I missed.
I've also found that some laptops open from the top, like my roommate's dell and my old, and others open from the bottom, like the new MBP Unibody. Maybe turn yours over and try from the other side?