If you design well, there should be no problems re Mac vs PC. In theory, at least.
But there are always some little cross-platform gotchas, so just be sure that you check all the pages and functions on as many platforms and browsers as you have available before releasing it into the wild. Just like you should with any other website design.
We're a mostly-Mac shop, and have had very little problem with Zen on either platform. One of these days I need to get a Linux install and see how they do there!
BTW, there are sites where, for a fee, you can log in and check your work on a vast array of browsers and platforms. The name of one is browsercam.com (I have no connection to them, and to date have not actually used their service). A friend found a site where a bunch of smaller designers got together to buy an annual subscription, saving a lot of money.




