Luke from iFixIt.com tested the new MacBook Pro with 8GB of RAM and posted the following on MacRumors
We did some more testing and found some interesting things. We have not succeeded to go beyond the 4GB limit with any OS X GUI app. With a C app (eatmem) that does not use Apple's APIs, we were able to allocate 8 GB and have it reported in Activity Monitor (and top).
The C app was able to allocate up to 8 GB without paging to disk. However, OS X is not happy running at or above the 4 GB limit. Performance is very erratic, and we crashed OS X and Parallels multiple times.
8 GB allocated, the system crashed shortly thereafter.
Interestingly, when we booted Ubuntu on the machine it only reported 3 GB memory total. We don't have an explanation for that.
Overall, our testing showed that the system is unstable at 8 GB of RAM. Parallels takes forever to load, even when using
We then yanked a chip to do a comparison with 4 GB. With a single 4 GB chip, everything seems happy.
We then added a 1 GB chip for 5 GB total. We were able to get GUI apps to use all 5 GB and the system hasn't crashed on us yet. It will take more testing to determine how stable this configuration is.
We suspect that this testing implies a two things:
1) The hardware can handle a 4 GB chip without any problem
2) There are OS-level limitations with 8 GB RAM on these systems.