Using every last byte available is often not a good idea – when you need an extra VDM or space for snapshots you are stuck

SATA for data drives ? SATA drives will do about ~80 IOPS so plan accordingly. If might want to put OS on the SATA LUNs, as long as your OS does not use the page file a lot ( VM's get plenty of memory) and use the SAS drives for your DATA drives. That's where you would install applications and place application/database files.

That helps..  Now I am looking at about 15 to 20 guest machines spread over 3 ESX hosts. I was thinking of seperating the OS and data drives to the SAS and SATA drives respectively.

Now would it be safe to create a 1TB file system on the SAS (2.2TB total) and a 4TB File System on the SATA (8.2TB total) for this purpose and the create different ISCSI luns on each file system?  Just wondering if one file system will be optimal or do you need to have multiple file systems.

Appreciate all the insight.

This is what the EMC technicians spec'd out for our company.  They did a discovery of our servers and IOPS and they came back to us with this configuration. I have 12 300 gig SAS drives and 12 1TB SATA drives in a DAE.

Again this was recommended directly from EMC based on the data they gathered during their discovery, so thats what we went with.