At the time of this writing, Google seems to be using IP filtering to prevent people with non-US IP addresses from setting up accounts. Fortunately, “mgoebel” of the Internet Tablet Talk forum has figured out a beautifully simple way around this.
Here’s how it works:
When you receive your Google Voice invitation, send it to a someone you trust in the United States, and ask them to set up the account for you. You’ll have to trust them with your password, but you can change that later. Have them set up the account and send you the login credentials.