Similar problem here. Two apps I purchased and installed today through iTunes' App Store ("Units Conversion Wizard" a.k.a. "Unit Convert" and "Currency Converter Pro" a.k.a. "Currencies") took the icons of two previously installed, completely unrelated apps ("Eniro" and "Facebook", respectively), only with a slight twist: the two "copied" icons are grey. I am no longer sure, but I
may actually have installed both the two earlier apps through the iPhone's App Store, not through iTunes.
No "light" versions of any apps are involved.
All four apps seem to otherwise operate normally.
After deleting and reinstalling the two new apps a few times, I got "Unit Convert" to revert to its own icon. This only lasted a short while, though.
"Currencies" however still persisted in having a grey "Facebook" icon after several attempts at deleting and reinstalling.
What I did then was delete all four apps, then hard reset the device, then reinstall "Unit Convert" and "Currencies", then hard reset a second time, intending only
then to reinstall "Eniro" and "Facebook".
However, while both "Unit Convert" and "Currencies" had their proper icons after I'd reinstalled them, both had reverted to the improper, grey icons after the second hard reset - and N.B. that this was when the apps they "borrowed" their icons from weren't even installed on the device!
I then attempted the same procedure only without the second hard reset. After this operation, all four apps have their proper icons, at least for now.
Let's see how long that lasts...