Also I got a very detailed response from a guy over at Creative Cow (a great resource, by the way) that I'll share here in case anyone else can use the info. He is talking about scanning my pen and ink drawings for use in an animation being composited in After Effects for HD. Thanks. :)

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Do 1920x1080. That way, if anyone wants 1280x720, you're already set.

Now, regarding dpi: you want 1920x1080 pixels. DPI is PPI (the terms are interchangeable), which is pixels per inch.

To find the desired dpi for scanning, you shouldn't look for a standard DPI, since there isn't one for video work. DPI is only relevant when scanning print work to digital, or printing digital files to paper. When dealing with video, all you care about is pixel sizes such as 1920x1080.

But we need to convert the paper (inches) to pixels (digital file). The answer lies in the size of the paper image, and a bit of math.

If the paper were 16 inches by 9 inches, then to convert that to a digital file 1920x1080, we'd have to scan at this dpi: 1920pixels/16inches = 120 pixels/inch, or 120dpi.
You see? Digital pixels / paper size = scanning dpi.

Note that I conveniently chose a paper size which is 16x9 inches. In your case, you should only use part of your 11x8.5 page, so it matches the 16x9 HD ratios. So you should only use this portion of your page: 10.67x6.00 inches, or 11x6.1875 or thereabouts. Let's say you were using the second one. Use the 11" measurement to calculate the dpi:
1920pix/11 inches = 175 dpi approx.

If you have to choose a higher dpi, no big deal.