You go belly up and none of your heirs has the passwords to your accounts, it's probably going to be a nightmare to straighten out.
Timely reminder. By now everyone knows that the one important document you NEVER put in a bank safe-deposit is your will ... but anything involving passwords is exactly analogous and there's no standard practice yet. Do you have a lawyer? Write down all passwords and leave a copy with him/her.
Food for thought: Do the major password-using organizations have established procedures for what to do when a member dies and their heirs don't know the password? Is the internet littered with Facebook pages that can't be taken down because nobody has access-- and may not even know they exist?
Look up your local laws on intestate succession. Then you can decide whether you enjoy the prospect of your survivors squabbling over your domains. (Depending on your family dynamics, it's possible you like the thought ;))