Smartphones and tablets have plenty of computing power to host malware, and they are simultaneously connected to the Internet via a cellular connection and to the LAN via Wi-Fi. So everybody in your organization has a device capable of by-passing your firewall in their pocket.
The good news is that smartphone OSes were designed recently enough that their creators were able to build security into the platforms using techniques like ARM TrustZone, and “chain of trust.” Technologies of this type are merely optional on PCs. Plus,the Android and iPhone app stores tightly control the applications that they distribute, and most people don’t take the trouble to avoid this protection. With these system-level and application-level protections, smartphones and tablets are intrinsically less vulnerable than PCs.
On top of these traditional threats, a new one looms: HTML 5. Adobe Flash is so notoriously vulnerable that Steve Jobs refused to let it onto the iPhone. Adobe has now thrown in the towel, and committed to HTML 5 instead. HTML 5 is presumably safer than Flash, but it is untried, and it has powerful access to the platform more akin to a native app than to traditional HTML.
This means that we can expect a rising tide of smartphone-related security breaches.