iphone 4 using cellular data when on standby connected to wifi

iPhone 4 using Cellular Data when on standby and connected to wifi.
location: apple.com - date: July 4, 2010
This is a real strange one. I have two wifi networks, one at home and one at work. I have recently upgraded to the iphone 4 so I have lost my unlimited data tariff so I thought I would have a look at what data was being consumed on the phone. I have noticed that although I am connected to wifi on both networks, if I just leave the phone so the screen goes black and it goes into its lock mode, when I press the home button to use the phone, for a second or so it shows 3G as the connection and then after about a second the wireless logo appears. This is sort of giving the indication that when in sleep mode it is dropping the wifi connection until I start using the phone again and wifi kicks back in. Sure enough if I then check the cellular data usage it has increased. Sometimes just by 1 or 2 kilobytes on sent and received, sometimes 20 or 30 kilobytes or more on both sent and received. This is without receiving any e-mails or any apps in the background running. This is happening on both
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