What can a mainframe do that a networked cluster of servers cannot?

[Sporkman]

Just wondering after reading this article:

http://datacenterjournal.com/index.php?option=com_content&view=article&id=4022:mainframes-today&catid=25&Itemid=100126

Is a mainframe just an efficient bulk purchase equivalent to a server cluster, or is there more to it?

[Sporkman]

I guess this sort of answers it:

http://en.wikipedia.org/wiki/Mainframe_computer#Characteristics