How complex can my bitcoin wallet encryption key be? What are the requirements and limitations of the password that the satoshi bitcoin wallet allows the user to set?
The password must be at least 1 character long.  I've not been able to find a maximum length.  I set the passphrase to be a 100,000 character string, and that worked fine.  Missing the last character off the end of the long string didn't work, so the 100,000th character is significant.

When first setting a passphrase the satoshi client requests that your passphrase be at least 10 random characters or 8 random words.  But it doesn't attempt to enforce its request.