Very much not so, even less square than 4:3.
Best way to always check these things, open Vegas project, set properties to match whichever format your working in, make certain preview has 'Simulate Device Aspect" on and note the frame size. This is the size to use for square pixels as in PS (except for CS if you use non square pixels).
Never failed me.
Bob.
In Photoshop CS, the new image dialog box has presets. An easy way to setup a widescreen image. Using the preset also will enable pixel aspect preview if different.