"As ubiquitous as Wi-Fi networks are, they're not everywhere, and open Wi-Fi access points designated for public use are becoming more and more rare every day. You can pay to subscribe to Wi-Fi networks, but that can be expensive over time - as can buying latte after latte at your local coffee shop. The cellular networks will sell you a USB or ExpressCard-based wireless modem for anywhere Internet access, but they'll charge you an extra monthly fee (usually in the $50 realm) for the privilege of this service. There's a better way, and it involves using your Windows Mobile phone in a way you might not have considered before: as a wireless modem."
If you're new to tethering, check out this informative column on the Windows Mobile web site. It just happens to be written by our very own Jason Dunn, so if you have any queries or comments, feel free to fire them off in this thread and I'm sure our head honcho will attend to them, as always.