I have similar "corrosion" on my bike. The bike is definitely aluminum (199? Trek 1420 w/ "Aluminum" in a big ol' decal on the top tube).
On my bike, it's where the top tube meets the headset, right around the hole where the rear brake cable goes into the top tube. The paint/clearcoat is fine, it just feels and looks rough, as if something is happening from the inside-out.
I too am concerned...I don't want my frame to come apart while I'm in a turn doing 20mph.
What's causing this??