Will Chrome OS work with Macbooks in the future?
Or is it only for PCs?
Only Google can properly answer this question but there isn't any technical reason it wouldn't work on a Mac with an Intel CPU.
EDIT: Just to clarify why I'm saying Mac's with Intel CPU. I don't have any knowledge/experience of Mac's with PowerPC CPU's so can't really comment. It might work fine on all Mac hardware.
Not currently, as the current Chrome OS builds are very targeted to specific hardware profiles. They minimize the number of third-party drivers (unlike normal Linux distributions which tend to maximize third-party driver support) so there will like be major issues with wifi, ethernet, and video card support. (The CPU will be least of your worries!)
It is extremely unlikely Google will ever release a Chrome OS downloadable build, akin to standard Linux distributions. However, you are more likely to find someone who has created custom Chromium OS build with the missing drivers installed on a custom kernel. Alternatively, if you don't mind losing some hair, you can invest time yourself learning how Gentoo and the Chrome OS build framework works; everything is documented at http://www.chromium.org/chromium-os/developer-guide.
Personally, I've ported the NVIDIA drivers to Chromium OS for a side-project, though Google will never accept it as a patch as it is closed-source binary-blob drivers. Theoretically this could provide support video on most Mac Book Pros and Airs.
Of course, regarding the other components, Linux drivers need to actually exist. Unfortunately, I've found with my early-2011 MBP there /are no/ Broadcom wifi drivers available yet, so even if I boot Chromium OS (which I have done) I would be SOL trying to get anything over the air. I didn't even attempt any of the other low-level drivers.
YMMV, while this would be a very cool hack, you'll probably have a much easier time trying to get OSX installed on a Cr-48 ;)