Eh-hem. It SHOULD work with cable since they will be launching their whole "online gaming" thing soon.
It most likely will use MSN.
It has a HDD, I don't see why you can't instal Winamp and play MP3s (Even better than that, supposedly, the XBox makes piracy of music rather simple. Something like ripping songs from a CD to play them while you play HALO, since there is no other decent game for the system. Anyway, network it to you computer, transfer files, and bam, I suppose you have a CD to burn now).
Don't know about the ICQ thing, but hey, once again....It has a HDD, and it is just basically, a computer....So, why not?
I wouldn't bet on being able to use a PS2 mouse. My cousin got the Linux kit for the PS2 (mouse included) and it hates the Microsoft mouse he tried to use on it once.
Most importantly, don't buy an XBox because Bill Gates sucks and so do all his products. Your computer hates you for installing Windows on it. And we wouldn't want to support the monopolizing franchise. Did you know Microsoft is losing about $2 billion over the course of the next 2 fiscal years on this experiment? Oh, but they have Halo and other good games like....Well, there's....I mean....No....