Valve updated Portal a few moments ago with this cryptic line being the sole change.

Portal

• Changed radio transmission frequency to comply with federal and state spectrum management regulations

Don’t ask me what this means, but there’s a new achievement for the game, and Steamforums posters are already all over this. For now all we know is that there’s rumors of a radio in each chamber, and they play different messages, in morse code or whatever. As an European, I can’t check it out, as Valve updates always come at the darnedest times.

EDIT: More decodes. Cheers for the link, Lunch.