Given your preference for .Net [CF] you are basically asking for a Windows Mobile or Windows CE device - depending on your specific needs anything that runs WM5 or WM6 should do what you want and be relatively cheap. Windows CE tends to crop up on more industrial type kit but there is a lot of cross over.
Symbol (now part of Motorola) makes some very good lightweight commercial handhelds, I often see them used as mobile POS systems, and they also have some very nice heavier industrial handheld type systems. Intermec also make some handheld industrial kit and I've found them more robust than the Symbol's but not as easy to manage. Both these vendors provide additional platform and solution development kits (at a cost) for their devices. These devices are not cheap - typically they go for between $600 and $3k depending on how rugged they are and the extras you choose.
Any current gen Windows Mobile Professional device could meet your needs but you definitely
need to check out the user interface, especially how well the touchscreen performs for your task. Almost all WinMo Professional devices use resistive "touchscreens" that require the use of a stylus to be accurate, that will have a significant impact on your UI design choices. Depending on the data input requirements you may want a device with a keyboard\thumbpad. In all cases I'd strongly recommend that you test them to see if the layout and design actually improves usability, I've seen some very poor implementations. Avoid anything with "Windows Mobile -Smartphone Edition" in the name, those are non-touchscreen phone devices (The Samsung Blackjack would be an example).
WLAN is one option but setting it up securely with handheld devices can be painful - the Wireless stacks are not as stable\mature as they are on PC's, they get stressed a lot more since they actually have to dynamically roam between AP's far more often, they suspend and resume more often and power consumption is always an issue. Secure provisioning of WLAN configurations on handhelds is time-consuming and generally difficult but there are approaches that can be taken. Debugging WLAN connectivity issues on handhelds is a lot more difficult than it is with PC's and I've often seen cases where Notebooks work fine and all the handhelds fail. To be fair though the WiFi stacks on current gen devices are much better than they used to be, that is one reason to be cautious about older hardware.
WWAN (cellular) is another possibility depending on your device choice and may actually be a lot simpler if you are in a position to take the cost and have appropriate coverage in the locations you need. If you don't already have a managed WiFi network, and security is in any way important to you then using a VPN Client over one of the 3G services may be more practical, just make sure that you actually have a workable VPN solution before committing to this, you certainly can't assume that there will be a VPN client available for your device or that your VPN infrastructure will play nicely with it.
If you aren't tied to .Net then there are a lot of other options - if you can work around the App Store issues (e.g. by delivering your app entirely via a browser) then the iPod Touch could be a good solution. WiFi compatibility and performance on both the iPhone and iPod touch are pretty good. Apple themselves have recently moved to Touches for their mobile POS (or are in the process of doing so) but they used Symbol devices running WindowsCE\WinMo for the last few years.