Noticed this for the first time today when I ran EveryTrail Pro. I have not noticed it in any other apps yet. Anyone know what this additional icon right up against the WiFi icon is?

If you have upgraded to OS 4.0 and the app is working for you, then you have an app that will run in the background. You can test this by closing the app, and you should see an indicator next to the battery indicator to let you know that GPS is being used.
From their FAQ:
Did you look on their website?
From their FAQ:
From their FAQ:
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Actually now that I look at your pic it's not next to the battery indicator. But I think it's GPS.
Must be Photoshopped. Either that, or SkyNet is finally becoming self-aware.
Is the app gyro enabled?
GPS signal strength indicator
in the app, goto: more/settings/GPS Settings. Scroll down and you will see the toggle for the signal.