Er... Not quite. Currently, the MBP, MB and even the iMac lines all use variations of the i945 "Napa" chipset, which is 32 bit. In a 32bit memory addressing environment, there is a maximum address range of 4GB. This address range not only has to account for all usable RAM, but also any other memory and devices that access memory within the system. Including the video memory and hardware I/O addresses. The reason why Apple only sells 3GB in the iMac C2D and not 4GB is because if you place a full 4GB in there, you would only be able to actually use about 3.2GB of it. This is the same for any notebook systems from Dell and anyone else or any other Merom C2D based system right now for that matter. The reason Dell and some PC makers sell 4GB in their systems is so that they don't lose any performance by only installing 3GB. The current chipsets support dual-channel memory access, but for this to work, the modules must be matched in size and specs. Apple is only offering a mis-matched 1GB+2GB max config in the iMac C2D. By doing this, they're sacrificing the dual-channel performance advantage (which isn't that great of an advantage), but they're also not selling RAM you can't use. I think I have to agree with Apple and say that it's more fair to the consumer to only offer 3GB than it is to offer 4GB and explain to them that they can only use about 3.2GB of it.
There is a 64bit version of the Napa chipset for workstations, (Napa-64) and it supports up to 16GB RAM. But no mobile version unless Intel/Apple have come up with something special. The next major revision of the mobile chipset (Crestline) will have support for at least 8GB, maybe 16GB. Reports are coming out saying it's ahead of schedule, but still guestimates are indicating that means February. Crestline is the chipset for the Santa Rosa platform.
I'm really hoping for a NAPA64 chipset in the updated MBP. I suppose it's possible as it's been mentioned from time to time in various PC forums and here. But realistically, we won't see more than 3GB usable in the MBP until the Crestline CPU arrives... I suppose it could be real early and show up in the next few weeks, but I seriously doubt it. I'm also starting to wonder if Crestline is early though and Apple is just holding out planning for a MWSF announcment and late January release.... I guess we'll know eventually, eh?