OK, I've fairly certain that the wifi is working correctly. Dell has send me the DW1530 so I installed the E6420 driver for DW1530 and it is problem free.
Conclusion: for some reason Dell is shipping the E6420 with the DW1501 wifi card and not the one listed on the driver page for the E6420. So if in device manager you see the DW1501 install the driver for the E6410 for DW1501 and if you have the DW1530 install the E6420 driver for the DW1530
I have a problem similar to this too. It's been happening since the first day I got my Latitude E6420. It's a certified refurbished machine with Windows 7 Professional, 64 bit. Whenever I restart the system the wifi icon says it's connected, but I can't get internet access until I run the home group troubleshooter, and the fix it applies says "home group information needed to be refreshed." A few weeks ago I downloaded and installed some updated drivers, but now the problem has escalated. When I open the laptop after it's been in sleep mode, there is no network connection at all until I right click the icon and click connect. About an hour or so ago, I looked for new network drivers, but I couldn't find any that matched my current drivers.