When you virtualize you still want to follow best practices (i.e. data, log, backups and tempdb on separate drives). However, this is where understanding the abstraction of resources becomes critical to understand.

When you virtualize you can create a separate VMDK/VHD (read also: virtual disk) for each drive. The OS sees each one as new drive, you mount them and letter them so now you have your C, D, E, F, T drives. The problem comes in is if all of those VMDK/VHD files were cut from the same LUN, then you haven't gained anything since they're all going to be sharing IO which is bad. Also if that one LUN goes out then you lose everything, again, negating your perceived benefit.

In order to truly "do it right", each VMDK/VHD should be on separate LUNS and those LUNS shouldn't be shared with any other IO-intensive applications (such as Exchange). This is where you need to become REALLY good friends with your SAN admin and come up with something that works. Hope this helps!