I am planning our Exchange 2010 Mailbox server. Can the RPM speeds of the hard drives differ for the drives used for the DB LUN and the Transaction Log LUN?

Yes, they can. There is no requirement for them to be the same RPM or even the same type (SAS/SATA), so long as they can support the IO performance your solution requires. SteveSteve Goodman Check out my Blog for more Exchange info or find me on Twitter

Make a note: I/O performnce has been increased by 70% in Exchnage 2010 server. You can go ahead with cheap disk like SATA.Anil MCC 2011,ITIL V3,MCSA 2003,MCTS 2010, My Blog : http://messagingschool.wordpress.com