Exchange 2007 isn't supported (by Microsoft) on VMWare for one.  I would
expect a supportability statement for Exchange on Hyper-V real soon though.

If you can live with the above, then take a look at the roles.  If your
organization is large enough that you're splitting out the Exchange roles, I
could see HUB and CAS virtualized no problem.  The mailbox role with a lot
of mailboxes is going to be storage and memory intensive.  As you get bigger
and beefier servers, the cost increase isn't necessarially linear.  You'll
have to carefully consider the one big box plus VMWare licensing  (or
Hyper-V assuming supportability) vs. the cost of more smaller boxses (don't
forget to add the costs of power, cooling, and rack space, etc. just to be
fair here).

For Storage, I'd strongly consider a SAN solution.  The DAS vs. SAN decision
is independent of any server virtualization decision.  Most SAN vendors pool
the storage and allow you to carve it up, so you can reduce or eliminate the
"islands of waste" problem of DAS.  If it's Netapp, and you're leveraging
VSS for backups, the hardware VSS provider has virtually no performance
penalty.  You can have up to 255 snaps in place per volume with only 3%
performance degradation.  Compare that to Copy on Write providers, like the
MS software provider, where with as few as five snapshots in place
performance degrades to the point where the storage is unusable.  For
Exchange 2007, application level replication is certainly available to you
for the price I like (free), so take a close look at CCR and/or SCR.
Compare that to the replication your hardware vendor offers, and how well
each of them meets your requirements.  A lot of times you'll find that the
Exchange native application level replication will do, other times you'll
find it won't.  It all depends on your specific requirements.

Believing VMWare or other Virtualization technologies can't break.

In all honesty you need to look at this from all angles, including cost.

A CCR Cluster for example will be two seperate nodes, with two seperate DAS
devices (more than likely) connected. If one breaks the other has no
dependancy.

Certain VMWare installations share storage, so if you single point of
storage breaks (no point in having a VM CCR cluster with both nodes pointing
to the same LUN right?) you're still going to have issues.

There's also the supportability stance, Microsoft may not support you as
well as if you were running physical hardware.

There's also the argument in how well does this virtualization
partner/consultant understand Exchange.

Oliver

Sure, virtualization is an excellent idea. For the HT, CAS and Edge
it's a no brainer. When you look at virtualizing the mailbox servers
there are a lot of additional things to consider and a shift in
thinking.
Rather than scale the server as far up as you possibly can - shoving
several thousand users and the full 50 stores on it you should have
more guests and keep them down to 8 (or so) stores. Why? Simple. if
you want to use Vmotion you need to have enough resource on a
candidate target to move it. Having a hulking great guest that needs
24 or 32GB of memory severely restricts your options as to where you
can move it. Either that or you end up not utilizing the ESX server
efficiently enough because you're maintaining all that headroom just
in case you need to re-home a big fat guest.

Here's the contentious part. Obviously for this you cannot use
Hyper-V. The only way you can properly, reliably and efficiently
virtualize a mailbox server is with VMware attached to a SAN. The MS
model where you provide Hyper-V resilience with a cluster does not
lend itself to virtualizing mailbox servers.

Then of course you need to wait for the support model to come out and
make the necessary payments!

And that's my 2 cents worth of opinion, take it with as large a pinch
of salt as you wish.

If you are looking at VMWare try this site

http://www.vmware.com/landing_pages/exchange_solution.html