The only differences appear to be cosmetic: the LT is purple with a black base, the HD is black with a purple base. But the electronic innards, features, and performance of the two boxes appear to be identical.
The other distinction between the LT and the HD is one of availability. The LT is intended as an online-only product, while you can find the new Roku HD at brick-and-mortar retailers such as Wal-Mart, Target, and Fry's (though it's available online, too).
So, if these are basically the same products in different packaging, why does the Roku HD cost $10 more than the Roku LT? Your guess is as good as mine. That said, the LT is so popular that it's often out of stock at online retailers like Amazon. If you encounter that problem -- especially if you're buying one as a gift -- spending the extra on the Roku HD isn't a bad backup plan. In the meantime, we'll continue to consider these products to be more or less interchangeable.