Depending on your budget, Terminal Services might be a good compromise. When they’re in the office, they use Outlook and when thet’re on the road, they use Terminal Services with Outlook installed. Granted, this can get to be very expensive to do and there are a lot of different ways to do it (RAS Pool, Citrix Secure clients, SSL, Two-factor authentication, Certificate services, etc).
The best alternatives that I see are as follows:
1) RAS Pool (no joy – as Remote Outlook can be extremely slow)
2) SSL VPN (We have used Whale communcations before with good success) Here’s a link to a Network World evaluation:
3) IPSec VPN (more difficult to setup and likely to be more expensive than an SSL VPN)