Based on a little "research" today I'm still not sure about SqueezeBox vs. Apple TV. For streaming songs from an imac to a 990 the differences seem to be:
SqueezeBox doesn't require the TV to display menu/song playing (although TV might display more information)
Apple TV has a hard drive (could this prevent possible hiccup problems with wireless connection?)
SqueezeBox has a better DAC (not so important if using the 990 DAC)