Biggest issue against BYOD for a lot of companies is funnily enough cost and money.
Netowrk and systems infrastructure in most companies has been setup to secure laptops and PC's. Mobile OS's and devices were never a design consideration.
To update your infrastructure to support the severely increased data leakage exposure that comes with tablets and smartphones requires a large investment in technology and training for your support staff.
I have been reviewing and comparing MDM's for several months now and have still not found one that adequately manages all mobile OS's out there. Only way I see to do this is with a proscribed list of devices provided by the company. BYOD would be a nightmare to support with multiple versions of Android OS, Windows phone 7.0 and 7.5 not supporting encryption just some of the problems.