I'd be remiss if I didn't talk about Research in Motion's (RIM's) BlackBerry device, a handheld gizmo that looks like a high-speed collision between a pager and a calculator. The product has a full (albeit tiny) keyboard and incorporates most of Microsoft Outlook's functionality: contacts, email, tasks, and a calendar.
To use the device, you install a BlackBerry agent on either a desktop computer or your Microsoft Exchange Server machine. If you install the agent on one machine, you can obtain mail from one mailbox; the server-based version can deliver mail from as many mailboxes as you'd like. The agent pulls new email from the user's Inbox, compresses the mail, and sends it across the Internet to the BlackBerry server, which broadcasts the messages (still encrypted, of course) to the user's handheld device. The result is completely wireless mobile email that integrates fully with Exchange Server. If you send a message from a BlackBerry device, the message will appear in your Sent Items folder when you return to the office.
Strictly speaking, this device isn't an Internet client solution, and neither the device nor the service is cheap. Also, the devices currently don't work in Europe or Asia-Pacific regions. However, if you're in North America and you want to provide a way for key people to get email when they're away from the office, BlackBerry is worth looking into.