Because my friend posed the question, I thought my email response would make for an interesting blog post. Reader's Caveat: I acknowledge that my response is anecdotal and my theories are not founded in thorough research. If you're still interested then, by all means, read on!:
Oh, boy. You've really asked the million-dollar question! I could go on and on about this...but I'll try to keep it relatively brief.
I've seen the article you referenced (a black family who adopts a white child)...and have not seen any other since. I do know of one blogger, atlasien, who writes about her experiences at Upside-Down Adoption because she self-identifies as an Asian "hapa" and adopted a non-Asian child.
Yes, the "trend" of whites adopting Asians (particularly Chinese and Korean) is probably the most common international adoption trend I've seen. There are SO many bloggers who prove this theory (see my Similar Blogs links). Unfortunately, I've never done a study on why this is. However, if you're really interested as to the "why", I'm sure that the Evan B. Donaldson Adoption Institute would have some answers. They are a great research institute.
Anecdotally, here are my thoughts:
I don't see many Japanese adopting white children because the closer they are to their Asian roots (meaning: if they have traditional Japanese families), the more stigmatized "adoption" is. They keep adoptions in Japan very hush-hush. When I lived in Japan, I stopped sharing with Japanese people about my adoption openly because it put them in a awkward position - they didn't know how to comprehend it. It also exposed them to a side of me that is very personal (which is not common for Japanese people to be so open with a private matter).
I think that China has set up a situation for abandoning girls (due to their one-child law and the socially-desirable child is a boy). That's why we see many Chinese girls being adopted by families here in the States. A similar situation happened in Korea, as women have out-of-wedlock pregnancies which are just now (slowly) becoming accepted by society...
On a larger scale in the U.S., the Multiethnic Placement Act (MEPA) of 1994 and Removal of Barriers to Interethnic Adoption Provisions (IEP) of 1996 attempted to provide equity in adoption for African American children because they saw the large number of black kids available for adoption and found a shortage of willing same-race families to place them in. The "color blind" interpretations of MEPA-IEP actually restricted agencies from providing post-adoption support for transracially-adopted kids. Look at the Evan B. Donaldson Institute's website for their recent statement in 2008 about how this didn't quite accomplish what the government intended.
Since this letter was written, I have since found an earlier article about Mark Riding found here in 2007 and a reference to another family in an archived Detroit News article here. But, even I have to admit, this black family-white child topic is hard to find.
I did find one interesting comment on Yahoo! Answers in response to the question, "Has a black family ever adopted a white child?". Check out Julie J's answer which takes into account the supply/demand in adoption.
I would love to hear your thoughts: why is transracial adoption not more of a two-way street? What do you think about seeing these new, "blended" families in your neighborhoods? Would they be well received?