CS Upgrade and platform change

So, I need a new laptop, not as a main machine. I'm deciding between the 13 and 15 inch macbook pro.

I also need an upgrade from CS3 to CS4. I had read that when you do an upgrade, you can do a platform switch.

Question: Can I keep using my CS3 on my windows box, and use CS4 on my new Mac if I do such an upgrade? I know it's probably not 100% kosher with their licensing, but will it WORK? Better yet, will CS4 activate on both?

As it is, the current CS3 license allows for 2 installs, which is cool. But I'm hitting the age old cross-platform question. Adobe really needs to open this up, it's such a hassle.