MSDN, as well as many other sources, claim that worker threads in the thread pool are always background.
"Thread pool threads are background threads
Thread pool threads are background threads
Finish that sentence with "they have their IsBackground property initialized to True, unlike threads created with the Thread class."
Setting it to false could be a bit risky. Threadpool threads are recycled, I'm not so sure that the property will be re-initialized. It is not a property associated with the physical operating system thread, they don't have IsBackground behavior, it is added by the wrapper that the CLR puts around it. So probably yes. Little reason to mess with it though.