We have been tailing the Yahoo! and Microsoft Search Alliance's movement. The first part of this global transition of Yahoo's search results to Microsoft saw Bing powering the Yahoo organic search results in the US and Canada. The next move was announced last week with 3 new countries – Australia, Brazil and Mexico being served by Bing.
The next country to be affected by the transition could be the UK. Although without any announcement on the same from Yahoo! or Microsoft, this guesswork is based on the experience of some webmasters and SEOs in the UK who are using the Yahoo! search. A Webmasterworld discussion revealed this interesting point.
According to the conversation, a person (in the UK) who used the Yahoo! search reported noticing two different indexes in the results which also rotated when the result page was refreshed. It means a search conducted twice gives two different results on Yahoo! in the UK. The same seems to be happening in the Bing search results too.
This strongly suggests that the Yahoo! and Microsoft Search Alliance is conducting tests in regards to their global transition program. Hence, if that is the case, UK might be the next country to be served by the new Bing powered Yahoo! search.