Yes - I have tested separating databases, transaction logs, tempdb, backups, swap space, snapshots, reporting vs. OLTP vs. SSAS, etc. with DASD and some SANs. In the situations I was in, disk was a bottleneck on the DASD and separating the processing types yielded benefits.
On the SANs it was a more complex situation. More often than not the SANs can handle the IO. In a few situations, the databases needed to be mapped back to their disks and we need to keep particular databases on disk separate from other databases. In a few situations the larger databases with more intense IO would cause performance problems for smaller less IO intensive databases. If you are faced with this situation you need to work with your SAN vendor and understand how their technology really works and figure out the best solution.