I'm curious to hear what other people have to say about your ds...we deal with aggression issues here, too, and as my boys get older I sometimes get worried that it's not just 'normal' boy aggression. Part of the problem here is that they're identical twin boys, so they have had to compete with each other for EVERYTHING since the day they were conceived.
They are only aggressive with each other, but are REALLY aggressive. Since they turned 2, they just keep finding new ways to hurt each other - biting, scratching, pinching, pulling hair...you name it, they do it. Sometimes the aggression just comes out of nowhere, like they just have this pent up negative energy they have to release. If I am close enough to stop the violence before it happens, ds will just stand there baring his teeth and shaking his clawed hands for a minute before calming down.
Other than these fighting episodes, they're lovely little boys - they're super smart (they started reading before they turned 2 and now read at a grade 3 level or above), generally well behaved in public, like to sing and dance, and can surprise me with their politeness at times.
Since I'm a SAHM they've never really spent lots of time with anyone but me. Now that they're in preschool 2 mornings a week I'm curious to hear their teacher's assessment of their behaviour - I figure only a non-family-member would be objective enough to tell me the truth if they think there might be a behavioural problem.