Because Sametime Mobile client protocols operate over IP, you could also use other VPN clients, but check to see whether your VPN vendor supports required handheld platforms. Microsoft's Communicator Mobile product also encrypts communications. There's no need for VPN software unless you're also trying to secure other mobile applications. Microsoft and IBM IM products do not interoperate with each other.
We're also beginning to see mobile IM gateways that you can install in your network. In this case, you use the gateway vendor's clients, which communicate with the gateway, which proxies the session to your IM server. NeuStar's Mobile Messaging Gateway and Sybase's One Bridge Messenger are two examples. See "Mobile IM Architectures" in the gallery, for a comparison of consumer-oriented mobile IM and enterprise mobile IM, with and without a gateway.
Buyer Beware
IM service providers and vendors may suggest hybrid approaches for mobile IM. Unfortunately, for most companies, they're not ideal. One method is to forward IM messages to users' phones using SMS. Public IM service providers let you configure this option using a Web page, but it's an awkward approach because SMS isn't session-oriented, and you'll lose benefits such as real-time presence information.
Another possibility is to use a mobile browser to log in to your IM service, where you engage in sessions through the browser. Compared with using an IM client, this is a slow way of interacting with the service because of wireless network delays, though longer sessions don't cost more since data plans are based on data usage or on a flat rate. It may be better than no access for occasional use or when a mobile IM client is not available, but in general, steer clear.