I'd say the biggest problem in mobile security today is the same biggest problem in any aspect of networking -- users. Users who don't follow enterprise policies, users you don't secure their devices, users who lose their devices, etc. The other perpetual problem is non-existent or vague security policies.
"The consumerization of IT. Employee-driven technology. Workforce mobility. Whatever the phrase used to capture the trend, companies increasingly have to secure their data and operations in the face of worker-owned mobile devices.
The issue will only become more prevalent: In 2011, 41% percent of workers used personal technology to access business applications, up from 31% in 2010, according to a March report released by IDC and Unisys. While the study included personal computers, a Nielsen survey found that consumers who owned a tablet were less likely to use the traditional computer."