UPDATE FOR 2010:
This blog post has certainly grown legs over the last few months. With so many existing owners contacting me about this, I feel compelled to offer more info. Yes, you will need permission to do any and all construction work in your unit. Yes, its likely that most condo boards will permit you to soundproof but you ALWAYS need to get permission. Here is another file worth reading about a retrofit:
As part of my daily routine, I consult with buyers who are thinking of buying into a variety of developments. In most cases, the issue of soundproofing comes up.
The bottom line is that even in the last few months SEVERAL of my ViP clients have complained about sound leakage in their new units.
The best advice I can give to ANYONE buying new (now or in the future) is to consider using a higher end soundproofing solution vs the standard level that the builder is offering.
I have heard everything from people being able to hear their neighbours talking and, in some cases, much more!!
If you are worried about resale value for this improvement...DONT! I have worked with everyone from DJs to Juno Award winning musicians and big CEOs...all of them value sound proofing and wish the original owner had used the right materials early on.
This is a MUST listen to link about what different levels of soundproofing can do for you....
Here is a reason that ALL builders may want to consider offering various levels of soundproofing vs none at all.
Lastly...studies have shown that people with better soundproofing tend to move less often....