Are Windows Phone 7 handsets sucking on the 3G teat when WiFi is available?

Reports are emerging that Windows Phone 7 (WP7) handsets are suffering from a bug where data is being sent and received over 3G even when a WiFi connection is available.

[UPDATE: Microsoft is now investigting these "phantom" data usage claims.]

Reports are emerging that Windows Phone 7 (WP7) handsets are suffering from a bug where data is being sent and received over 3G even when a WiFi connection is available. 

Here's an example of the issue that WP7 handset users are seeing:

I can confirm that I'm seeing unusually large data usage with my phone. Today, I visited two webpages that were here on HowardForums. Of my 50MB plan, I now have 34MB left and that's with the wifi given preference. That would mean either I used 16MB of data in 90 minutes, or that data was still being used when I was back on a wifi network. I am... irritated.

Some users are reporting that simply having the handset check for email every 15 minutes uses a whopping 3 - 5MB of data per hour!

There's no word from Microsoft on this issue yet, but if this bug is real it's a big problem for WP7 handset owners because it could end up costing them big bucks.

Anyone here noticed their WP7 handset sucking data when it should be using a WiFi connection?