As you might have guessed, a lot of the answers to this question would being with "it depends..." Because even with the advancements in the Cx platform over the years in the end you are sharing physical spindles which are potential points of contention.

That being said I have had great success stacking multiple LUNs in a single RAID group for databases that are not high performance OLTP type databases. As long as the application does not require the quickest response time and can tolerate it, I think it is appropriate.

An even better way to stack LUNs would be to implement metaLUNs and create multipe RAID5 groups and stripe your database and log LUNs across multiple RAID groups. I have also done this with a lot of databases that are not very demanding as well and have one set of RAID groups supporting five separate servers each with multiple data, log and tempdb LUNs. So... it all depends on what the application can tolerate