The discussion is going pretty well and the different approaches mentioned are good. One thing I would like to point out here that all the solutions revolve around the same principle and follow the logical query processing phase in sql server.

1. FROM

2. ON

3. OUTER

4. WHERE

5. GROUP BY

6. CUBE | ROLLUP

7. HAVING

8. SELECT

9. DISTINCT

10 ORDER BY

11. TOP

Pay attention, here GROUP BY comes before SELECT and DISTINCT. Of course this will apply independently to sub queries and virtual table expressions.

Following this I don't see any reason why GROUP BY over aggregate concatenation will not work in any scenario. So far I have never encountered an example of the "undefined". If anyone can post an example precisely explaining this nature mentioned in the KB article then it would be helpful for all.