Tables are only valid in XHTML when displaying tabular data.
Tables are often slower to load and can dramatically change how your layout looks. You may end up with a very rigid layout that won't look good on all monitors.
Slicing: Graphic layouts and techniques such as repeating background images mean that one small image can be downloaded from the web server only once and then be instructed (via CSS) to repeat, shifting the work load from the web server onto the client's computer.