It’s certainly a safe assumption to use tables because they are universally supported in all the popular clients.
However, some people may not realize that divs are fairly well supported as well and in certain circumstances, div’s can make life a whole lot easier! For example clean borders, margins, and padding are easier to achieve using divs.
We actually recommend a mix between the two depending on your design and layout.
Just be aware of one thing: You are NOT always safe when using basic tables nor divs. This is due to the fact that several clients use default style sheets that may add unwanted margins or image spacing to your layout. In those circumstances it is best to use inline styles to override the default css.
It’s always good to test no matter how simple you think your design is! For a good example of what we mean by unwanted image spacing, checkout this post: http://www.emailonacid.com/create-html-email/details/C13/hotmail-style
