SQL Server - Separating data, log, and TempDB files on a SAN
One thing to consider is that the log files are sequential writes where as the data files are non sequential.  That is one of the reasons for separate LUNs.  Log files write faster if they are on their own LUN because the spindles don't have to skip around, just write sequential.  If you add in a data file then the spindles have to skip around and you lose some performance.  I'm hoping I got the right terminology there as I'm not all that familiar with SANs themselves.  The idea behind it should be however.

Frequently vendor recommendations are wrong when it comes to SQL Server.  Just because SQL Server has different needs than most applications that use a SAN.