The nice thing about HTML is that it is operating system independent. So this
means that it will be the same whether you create it on Mac or Windows.

The differences you may notice though are in the rendering of the pages.
Previously in MX there was still some styling done via the HTML code. Now CS3
has much more of an emphasis on CSS for styling ( see tutorial if you are
unfamiliar - http://www.w3schools.com/css/ ). So DW may create slightly
different code. While the code is CS3 using CSS is correct sometimes the
design view of the older version may look slightly different then the design
view of the newer version although the site will look correct when it is
uploaded. This is just because they have modified the design view slightly to
accommodate for the changing in web standards. Nothing too much but just so
you don't get scared if something looks slightly out of place.

But for someone starting out with DW, most people will recommend that you read
over the above tutorial just to get a grasp on CSS. And don't be afraid to ask
questions if you run into any while setting up and getting ready. And if you
are wondering I use a Mac and there are plenty of Mac users on this forum to
help you out if you need it.