When it comes to design and layout, you have physical dimensions and orientation. In CSS orientation is determined by the height to width ratio of the display. If the browser window is taller than it is wide, portrait is assumed (a monitor placed in a vertical orientation). If the browser window is wider than it is tall, landscape is assumed (most monitors in their normal orientation).
With the marketplace for tablet devices going well beyond the defined dimensions of the iPad (1024 by 768), versus something like the iPad mini (still 1024 by 768, but in a smaller physical size), versus all the variations possible with Android and other platforms...I would say it depends. You may want to consider researching target audience, user, etc. and not fall into the trap of assuming, just because you, the CEO, and the client have an iPad that all tablets are created equal.
There are other consideration beyond those of layout/design and size. With a tablet (mobile in general), the same assumptions regarding bandwidth should probably not be used. So, if your desktop design has 5 main HTTP requests to load CSS and JS files, and the page in question has 100 images between UI and content...you should probably design a different experience. Further, JS animations, for example, are dramatically slower/sketchy on many mobile devices; so, you may wish to design different interactive qualities.
A final design consideration - incorporating the information above - would be to design with the lowest possibility/capability in mind.
You can, generally speaking, divide handheld devices into 3 categories - small (most Blackberries, for example), medium (iPhone/iPod touch sizes), and large (some of the more recent Android devices). (Where you determine the cut-off to be is kind of up to you, and when exactly a handheld device becomes a tablet is, again, up to you, but there are sites out there with information on common sizes.) Also, chances are, these devices will access the site via a data connection over a cellular network.
You can, generally speaking, divide tablet devices into 2 categories - small (iPad Mini, Kindle Fire, etc) and medium (iPad, and others). After something becomes around 13" or more I tend to classify it as a monitor...but, that's just me...the main point is these devices are self-contained in that the display and the guts are one unit - unlike a laptop or e-book. These devices, will probably connect via WiFi; however, can also connect via cellular data networks - which should be a consideration when looking at the difference between the overall UX storyboards and wireframes you put together for desktop, tablet, and handheld.
Desktop, can also be divided into small (e-book), medium (most laptops), and large (monitors of a size roughly 19" or more) - you may even consider extra-large (projectors and televisions).