If you look at the iPhone, there are many examples on how to perform reachability network checks as well as best practices:
I’ve found the following existing questions on SO:
Where should I perform a Reachability check?
Apple developers, in addition, have an example of how to use their Reachability class:
Apple Code Example
I haven’t seen resources like this for WP7 yet. The AppHub’s example code shows projects that communicate to the web, but none (at least that I saw) that perform reachability checks. In addition, though Charles Pretzold’s free 1000 page PDF has many excellent examples and explanations, none of them concern reachability.
Has anyone done this yet or found solid examples?
UPDATE: Has NetworkInterface.GetInternetInterfaceType() been proven to work on devices?
If you’re wanting to know which network connection is currently active, you can do this.
NetworkInterface.GetInternetInterfaceType Method (Microsoft.Devices.NetworkInformation)
Not sure if that will give you what you’re after though. I understand you specifically asked for reachability via a chosen network connection, but figured the result you might be after in the end is to work out which network connection is active.. have a look anyway and see if it helps.