ActiveCollab can’t do that... you can comment files but not task or milestone. You can link from task to discussion though, it’s been a welcome addition.
Commenting on task and todos is a nice feature Ryan and I’ll try to get this one added to ActiveCollab (given the rate of improvement, wouldn’t be surprised if this happened). Not sure this will make it since Tickets (basically, extended tasks) can do that, Ilija might tell me to use tickets for complex tasks which require debating... we’ll see.
Anyway remember that BaseCamp (unless it has AT LAST hit the guys @37signals that internationalization could be of value to their product) is english only and that is a killer argument in favor of ActiveCollab at least for us who work with non english speaking clients
How such visionnaries can lack interrest for "non english speaking market" is beyond my understanding (I had contacted them by email over 2 years ago about translating in french for them, but never had any reply)... or else they think interresting clients all speak english ?