Clustering VS. Mainframe

[Liberty Young]

I was just pondering the following question based on what David said
earlier today.
> 
> Lots of people would rather spring for a cluster then huge single
> machine now a days.
> 


Putting aside that it's very geek-cool to run or put together a cluster
of cheap x86 pcs, what are the reasons to have a cluster vs a single
machine? 

One reason TO have a cluster is that you can run your own clustering OS
and kernel; then again, the kernel and OS may fall shy of the ones built
into and for the single machine. 

I was also thinking that with clusters, it is very hard to amass a clone
army of the same pc for each node. You would HAVE to have a contract
with a vendor that states very explicitly that they would supply you an
exact replica for each node. Otherwise, there'd be difficulty  and costs
involved in maintance. For example, you couldn't just use a single
restore CD or a ghost image of the machine to install a new one or fix a
corrupted one. 

Disclaimer: These are just theoritcal thoughts. I'm not a large scale
administrator of any kind.