Nokia's response to Mary Jo Foley strongly hinted that the display panel was the problem (they basically gave the "Hardware restrictions" party line but followed with "but the display brings all kinds of other cool stuff!"). I wouldn't be surprised if it's around the amount of juice (battery and CPU) required to fire up the display, it may be more than the OS permits while the phone is in a sleep state (AMOLED needs more juice than the IPS in the 1520, and the Icon has a higher PPI than the other AMOLED devices in Nokia's portfolio, hence why the issue would be unique to the Icon).
Don't rule it out yet, "hardware restrictions" is the party line, we'll see what happens in the next few weeks... Chances are they'll figure something out, especially if Nokia plans to release a comparable handset globally.